Method for blocking miRNA

ABSTRACT

A method of reducing the abundance of a non-target micro-RNA (miRNA) that is part of a group of miRNAs is provided, including: (a) annealing a complementary region of a blocking nucleic acid to a binding site at a first end of the unwanted miRNA, ligating an adenylated nucleic acid adapter to the group of miRNAs, and performing RT-PCR on group of miRNAs. Kits and compounds for use with the method are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division under 35 U.S.C. § 120 of U.S. patentapplication Ser. No. 15/565,652, filed on 10 Oct. 2017, which issued asU.S. Pat. No. 10,480,023 on 11 Nov. 2019, U.S. patent application Ser.No. 15/565,652 is a national stage under 35 U.S.C. § 371 ofInternational App. No. PCT/US16/26846, filed 10 Apr. 2016. InternationalApp. No. PCT/US16/26846 cites the priority under 35 U.S.C. § 119 of U.S.Pat. App. No. 62/146,015, filed 10 Apr. 2015. The contents of theforegoing applications are incorporated herein by reference, althoughthe prosecution histories of said foregoing applications are notincorporated herein.

BACKGROUND A. Field of the Disclosure

The present disclosure generally relates to the field of molecularbiology. In particular, the present disclosure pertains to generating asequencing library of micro RNA (miRNA). More specifically, the presentdisclosure pertains to reducing the frequency of specific miRNAs in asequencing library.

B. Background

Micro RNAs are naturally occurring, small non-coding RNAs that are about17-25 nucleotide bases in length in their biologically active form.miRNAs post-transcriptionally regulate gene expression by repressingtarget mRNA translation and by targeting transcripts for destruction. Itis thought that miRNAs function as negative regulators, such thatgreater amounts of a specific miRNA will correlate with lower levels oftarget gene expression.

Given their important role in gene regulation, and therefore humanhealth, large scale sequencing of miRNA has become a very valuablescientific tool in the study of human disease. There are various methodsknown in the art of creating a miRNA library to be sequenced.

Small RNAs can be measured with a variety of technologies, includingqPCR, microarrays and solution-based hybridization, amongst others.Next-generation DNA sequencing (NGS) is also a powerful method for thediscovery and quantification of small RNAs due to its technicalperformance, low expense, ultra-high throughput and its ability toagnostically detect and measure new species.

For example, as generally shown in FIG. 1 , in a protocol utilized byIllumina, Inc. and other commercial companies that make Illuminacompatible kits, to generate a miRNA sequencing library, an adenylatedDNA 3′ adapter 40 with a blocked 3′ end is ligated to an RNA molecule's10 3′ end 20 using a truncated T4 RNA ligase 2 50. This truncated T4 RNAligase 2 50 requires the 3′ adapter 40 substrate to be adenylated. Theresult is that fragments of other RNA species in the total RNA sampleare not ligated together in this reaction; only the pre-adenylatedoligonucleotide can be ligated to free 3′ RNA 20 ends resulting in amiRNA molecule with a 3′ adapter ligated thereto 60. Moreover, since the3′ adapter 20 is 3′ blocked, it cannot serve as a substrate forself-ligation. In the next step, a 5′ adapter 70 is added along with RNAligase 1 80. Only RNA molecules 10 whose 5′ ends 30 are phosphorylatedwill be effective substrates for the subsequent ligation reaction. Afterthis second ligation, an miRNA with both 3′ and 5′ adapters ligatedthereto 90 is formed. Next, reverse transcription polymerase chainreaction (RT-PCR) amplification 100 is performed. After RT-PCRamplification 100 the library may be sequenced and analyzed. Thislibrary preparation method results in an oriented library such that thesequencing always reads from the 5′ end 30 to the 3′ end 20 of theoriginal RNA molecule 10

However, NGS of small RNAs has several technical challenges. Among theseis the well-reported biased behavior of the modified forms of T4 RNALigase 2 commonly used in sequencing library generation protocols. Thisbias manifests in small RNA libraries as differential ligation, creatingan over-representation of certain species and an under-representation ofothers. When small RNA libraries are constructed from many sample types,these biases in ligation efficiency, combined with inherent abundancedifferences, can yield inaccurate results. Highly abundant small RNAspecies may be preferentially ligated such that their representation inthe library becomes inordinately high, diminishing the ability tomeasure other less abundant species. The precise detection of theseunderrepresented species would thus require very high sequencing depthsand proportionally higher costs. Additionally, highly abundant speciesinterfere with many normalization techniques, limiting the utility ofthe collected reads.

In small RNA libraries made from human plasma and serum, many of themost highly abundant species are probably derived from blood cellpopulations. While these may be of interest in some applications, miRNAsand other small RNAs that act as biomarkers for many diseases, such ascancer and neurodegenerative disease, may be of low abundance in theblood of afflicted patients. Accordingly, the problem facing researchersinterested in blood-based miRNA biomarkers is how to measure preciselylow-abundance species in a background of highly abundant and lessinformative species that comprise most of the reads in sequencinglibrary.

Accordingly, there is a need for an effective method for reducing thefrequency of overrepresented or abundant miRNAs 10 in miRNA sequencinglibraries.

SUMMARY

The above problems (as well as others) are addressed by the inventionsprovided in this disclosure, although not every embodiment disclosedhere will address every problem disclosed above.

In a first aspect, a blocking nucleic acid for use in reducing theabundance of a unwanted micro-RNA (miRNA) in an miRNA library isprovided, the blocking nucleic acid comprising: a 5′ end of the blockingnucleic acid and a 3′ end of the blocking nucleic acid; asingle-stranded complementary region at one of the 5′ end of theblocking nucleic acid or the 3′ end of the blocking nucleic acid, thatanneals with a binding region at a first end of the unwanted miRNA understringent conditions, wherein said first end is either the 5′ end or the3′ end of the unwanted miRNA, and wherein the complementary region has aterminal end; a hairpin loop forming region adjacent to thecomplimentary region, the hairpin loop forming region having a ligativeterminal end; and a first blocking moiety linked to the terminal end ofthe hairpin loop forming region, in which said first blocking moietycannot serve as a substrate for ligases.

In a second aspect, a blocking nucleic acid for use in reducing theabundance of an unwanted miRNA in an miRNA library is provided, theblocking nucleic acid comprising: a Crick strand having a 3′ end and a5′ end; a single stranded complementary region at one of the 5′ end ofthe Crick strand or the 3′ end of the Crick strand, that anneals with abinding region at a first end of the unwanted miRNA under stringentconditions, wherein said first end is the 5′ end or the 3′ end of theunwanted miRNA; a double-stranded region on the Crick strand adjacent tothe complementary region, the double-stranded region comprising a Watsonstrand that is annealed to the Crick strand, the Watson strand having a5′ end and a 3′ end; a first blocking moiety linked to the 3′ end of theCrick strand, wherein the first blocking moiety cannot serve as asubstrate for ligases; a second blocking moiety linked to the 5′ end ofthe Crick strand, wherein the second blocking moiety cannot serve as asubstrate for ligases; a third blocking moiety linked to the 3′ end ofthe Watson strand if the complementary region is at the 3′ end of theCrick strand, or linked to the 5′ end of the Watson strand if thecomplementary region is at the 5′ end of the Crick strand, wherein thethird blocking moiety cannot serve as a substrate for ligases; and aligative terminal end on the Watson strand, the ligative terminal endlocated at the 3′ end of the Watson strand if the complementary regionis at the 5′ end of the Crick strand, or at the 5′ end of the Watsonstrand if the complementary region is at the 3′ end of the Crick strand.

In a third aspect, a method of preventing an unwanted miRNA fromparticipating in reverse transcription polymerase chain reactions(RT-PCR) is provided, the unwanted miRNA having a 5′ end and a 3′ end,the method comprising: annealing the complementary region of either ofthe blocking nucleic acids above to the binding site at the first end ofthe unwanted miRNA, wherein the first end of the unwanted miRNA is oneof the 5′ end or the 3′ end. The product of the method is also provided.

In a fourth aspect, a method of reducing the abundance of an unwantedmiRNA in an miRNA library is provided, the unwanted miRNA having a 5′end and a 3′ end, the method comprising: purifying RNA from a samplecomprising a plurality of miRNAs; introducing an adenylated nucleic acidadapter and a first DNA/RNA ligase under conditions to allow theadenylated nucleic acid adapter to ligate to the 3′ end of the pluralityof miRNAs; introducing either of the blocking nucleic acids above underconditions to allow the complementary region of the blocking nucleicacid to anneal to the binding region of the unwanted miRNA, to produce ablocked sample; introducing an RNA adapter and an RNA ligase underconditions to allow the RNA adapter to ligate the 5′ end of theplurality of miRNAs; introducing a reverse transcriptase to the blockedsample under conditions to allow the reverse transcription of theplurality of miRNAs to produce a cDNA sample; and performing thepolymerase chain reaction (PCR) on the cDNA sample to produce the miRNAlibrary with reduced abundance of unwanted miRNA. The miRNA library withreduced abundance of non-target miRNA that is the product of this methodis also provided.

In a fifth aspect, a kit for reducing the frequency of an miRNA in anmiRNA library is provided, the kit comprising either of the blockingnucleic acids above.

In a sixth aspect, a blocked miRNA complex is provided, comprising: anmiRNA; and either of the blocking nucleic acids above annealed to thebinding region at the first end of the miRNA, wherein the first end isone of the 5′ end or the 3′ end.

In a seventh aspect, a nucleic acid is provided, comprising a sequencehaving at least a certain level of identity to one of SEQ ID NO: 1-4 and13. In an eighth aspect, a nucleic acid is provided that anneals underhighly stringent conditions with the nucleic acid of the seventh aspect.In a ninth aspect, an organism or vector is provided comprising any ofthe nucleic acids of the seventh and eight aspects.

The above summary presents a simplified summary in order to provide abasic understanding of some aspects of the claimed subject matter. Thissummary is not an extensive overview. It is not intended to identify keyor critical elements or to delineate the scope of the claimed subjectmatter. Its sole purpose is to present some concepts in a simplifiedform as a prelude to the more detailed description that is presentedlater.

BRIEF DESCRIPTION OF THE DRAWINGS

A more particular description of the invention will be rendered byreference to specific embodiments thereof which are illustrated in thedrawings. It is appreciated that these drawings are not intended tolimit the scope of the claims.

FIG. 1 . Flow chart of an earlier protocol, in which a preadenylatedadaptor is ligated to the 3′ end of a small RNA pool using T4 RNA Ligase2, truncated. Subsequently, a second adaptor is added to the 5′ end ofthe miRNA with T4 RNA Ligase 1, followed by reverse transcription andPCR.

FIG. 2 . Flow chart on an exemplary embodiment of the method, in which ahairpin oligonucleotide with an overhang complementary to the 5′ end ofthe targeted miRNA is attached via ligation with T4 DNA Ligase to the 5′end of the miRNA subsequent to the ligation of the adaptor to the 3′end. This prevents the ligation of the second adaptor to the 5′ end ofthe miRNA, resulting in a product that does not amplify during PCR.

FIG. 3 . Graph showing the fraction of hsa-miR-16-5p present in ablocked library generated from human heart total RNA using a titrationof a blocking oligonucleotide targeting hsa-miR-16-5p compared to theunblocked library is shown on the y-axis.

FIG. 4 . Blocking of hsa-miR-16-5p in human plasma samples. (A-C)Sequencing results from five different human plasma samples are shown inA-E. Read counts from averaged (see Materials and Methods) replicateunblocked and hsa-miR-16-5p blocked libraries are shown on the x and yaxes respectively. All libraries were down-sampled to 6 million alignedmiRNA reads before plotting and analysis. A miRNA is consideredsignificantly differentially expressed between the two conditions if theadjusted P-value as calculated by DESeq2 is <0.01. Not significantlydifferentially, expressed miRNAs are shown as open circles.Significantly differentially expressed miRNAs are shown as filled blackcircles. (F) Sequences of the mir-16 family members are shown with theseed region (bases 2-8) highlighted in gray.

FIG. 5 . Effect of hsa-miR-16-5p blocking on read depth in human plasmasamples. A set of count thresholds is plotted on the x-axis versus thedifference between the number of miRNAs passing that threshold in thehsa-miR-16-5p blocked samples versus the unblocked samples is plotted onthe y-axis. The differences between individual samples are shown as graydashed lines. The mean difference is shown as a solid black line. Alllibraries were down-sampled to 6 million aligned miRNA reads beforeplotting.

FIG. 6 . Effect of hsa-miR-16-5p blocking on reproducibility anddifferential expression measurement in human plasma samples. (A)Dispersions were calculated for each set of plasma sample librariesbased on the replicate unblocked libraries and replicate hsa-miR-16-5pblocked libraries using DESeq2. The dispersion values are plotted on they-axis versus the base mean read counts, also calculated by DESeq2, onthe x-axis. The unblocked dispersions are plotted in gray while theblocked dispersions are plotted in black. (B) Fold changes werecalculated between all possible sample pairs (10) in both unblocked andhsa-miR-16-5p libraries. The log₂ (fold changes) for all of those pairsare plotted on the same axes, with the log₂ (fold change) for theunblocked library on the x-axis and the log₂ (fold change) for thehsa-miR-16-5p blocked library on the y-axis. Thus each point representsa unique miRNA-sample pair combination. Only those miRNAs for which bothsamples had a DESeq2-calculated base mean >10 were plotted. The mean andstandard deviation of the set of 10 Spearman rhos of the correlation ofthe fold changes between unblocked and hsa-miR-16-5p blocked librariesis listed on the plot.

FIG. 7 . Blocking of hsa-miR-451a alone and in concert with blockinghsa-miR-16-5p in human plasma samples. (A-B) Sequencing results from twohuman plasma samples are shown. Read counts from an unblocked libraryand a hsa-miR-451a blocked library are shown on the x and y axesrespectively. (C-D) Sequencing results from the same two human plasmasamples are shown. Read counts from an unblocked library and ahsa-miR-451a and hsa-miR-16-5p simultaneously blocked library are shownon the x and y axes respectively. A miRNA is considered significantlydifferentially expressed between the two conditions if the adjustedP-value as calculated by DESeq2 is <0.01 and if its base mean count isabove 50. Not significantly differentially expressed miRNAs are shown asopen circles. Significantly differentially expressed miRNAs are shown asfilled black circles. All libraries were down-sampled to 6 millionaligned miRNA reads before plotting and analysis.

FIG. 8 . Categorical distribution of reads from a set of unblocked humanplasma libraries. The fraction of reads falling into six categories for27 libraries derived from human plasma samples is shown. Solidhorizontal lines indicate reads aligning to miRNAs but not tohsa-miR-16-5p. Vertical broken lines indicate reads that map to thehuman genome but are not miRNAs. Vertical solid lines indicate readsaligning to hsa-miR-16-5p. Horizontal crosshatching indicates reads thatalign to the spike-ins. Checks are reads that failed to align to miRNAsor the human genome. Broken horizontal lines are reads that areadaptor-dimer.

FIG. 9 . Categorical distribution of reads from a set of hsa-miR-16-5pblocked human plasma libraries. The fraction of reads falling into sixcategories for 23 libraries derived from human plasma samples in whichhsa-miR-16-5p was blocked is shown. Solid horizontal lines indicatereads aligning to miRNAs but not to hsa-miR-16-5p. Vertical broken linesindicate reads that map to the human genome but are not miRNAs. Verticalsolid lines indicates reads aligning to hsa-miR-16-5p. Horizontalcrosshatching indicates reads that align to the spike-ins. Checks arereads that failed to align to miRNAs or the human genome. brokenhorizontal lines are reads that are adaptor-dimer.

FIG. 10 . Effect of the blocking ligation reaction when targeting the 5′end versus targeting the 3′ end. Plotted is the total libraryconcentration as determined using the Library QuantificationKit—Illumina/ABI Prism (KAPA Biosystems). The stippled bars arelibraries in which a mock blocking ligation (all reagents except theblocking oligonucleotide) was run as would be performed to block the 5′end of a targeted miRNA. The crosshatched bars are libraries in which amock blocking ligation was run as would be performed to block the 3′ endof a targeted miRNA.

FIG. 11 . Illustration of 3′ end variations in hsa-miR-16-5p effects onblocking efficacy by a blocker targeting the 3′ end. Shown are varioussequence variants of hsa-miR-16-5p, with the canonical form displayed asthe leftmost sequence. Together, the six plotted here comprise over 91%of the sequences aligning to hsa-miR-16-5p in this experiment. The barheight indicates the fraction remaining in the blocked library whencompared to the unblocked library.

FIG. 12 . Reproducibility of read counts in libraries with and withouthsa-miR-16-5p blocking. (A-E) Read counts for replicate unblockedlibraries from five human plasma samples are plotted versus each other.(F-J) Read counts for replicate hsa-miR-16-5p blocked libraries fromfive human plasma samples are plotted versus each other. For allexperiments, the aligned reads were down-sampled to 6 million beforeplotting. The Spearman rho coefficient of correlation is shown for eachreplicate pair.

FIG. 13 . Illustration of 5′ end variations in hsa-miR-16-5p effects onblocking efficacy by a blocker targeting the 5′ end. Shown are varioussequence variants of hsa-miR-16-5p, with the canonical form displayed asthe leftmost sequence. Underscores represent “missing” bases from thecanonical form. Variants shorter than the canonical form, and certainlonger forms show decreased blocking efficiency. Because these variantsrepresent a very small fraction of the total reads (<2%), it is unclearif the base calls represent true variants or sequencing errors.

FIG. 14 . Side by side comparison of abundance of hsa-miR-16-5p inunblocked library and blocked library using hsa-miR-15-5p 3′ blocker.

FIG. 15 . Side by side comparison of abundance of hsa-miR-26a-5p inunblocked library and blocked library using hsa-miR-26a-5p 5′ blocker.

FIG. 16 . Side by side comparison of abundance of hsa-miR-486-5p inunblocked library and blocked library using hsa-miR-486-5p 5′ blocker.

FIG. 17 . Side by side comparison of abundance of hsa-miR-16-5p inunblocked library and blocked library using pool of blockers.

FIG. 18 . Side by side comparison of abundance of hsa-miR-26a-5p inunblocked library and blocked library using pool of blockers.

FIG. 19 . Side by side comparison of abundance of hsa-miR-451a-5p inunblocked library and blocked library using pool of blockers.

FIG. 20 . Side by side comparison of abundance of hsa-miR-486-5p inunblocked library and blocked library using pool of blockers.

FIG. 21 . Effect of hsa-miR-16-5p blocking on reproducibility anddifferential expression measurement in human plasma samples. Foldchanges were calculated between all possible sample pairs (10) in bothunblocked and hsa-miR-16-5p libraries. The log₂ (fold changes) for allof those pairs are plotted on the same axes, with the log₂ (fold change)for the unblocked library on the x-axis and the log₂ (fold change) forthe hsa-miR-16-5p blocked library on the y-axis. Thus each pointrepresents a unique miRNA-sample pair combination. Only those miRNAs forwhich both samples had a DESeq2-calculated base mean >10 were plotted.The mean and standard deviation of the set of 10 Spearman rhos of thecorrelation of the fold changes between unblocked and hsa-miR-16-5pblocked libraries is listed on the plot.

FIG. 22 . An embodiment of the blocking nucleic acid without a hairpinloop in the method of blocking an miRNA. FIG. 22A shows 5′ blockingnucleic acid. FIG. 22B shows a 3′ blocking nucleic acid.

DETAILED DESCRIPTION A. Definitions

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art of this disclosure. It will be furtherunderstood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the specification andshould not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein. Well known functions or constructions maynot be described in detail for brevity or clarity.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise.

The terms “first,” “second,” and the like are used herein to describevarious features or elements, but these features or elements should notbe limited by these terms. These terms are only used to distinguish onefeature or element from another feature or element. Thus, a firstfeature or element discussed below could be termed a second feature orelement, and similarly, a second feature or element discussed belowcould be termed a first feature or element without departing from theteachings of the present disclosure.

With reference to the use of the word(s) “comprise” or “comprises” or“comprising” in the foregoing description and/or in the followingclaims, those words are used on the basis and clear understanding thatthey are to be interpreted inclusively, rather than exclusively, andthat each of those words is to be so interpreted in construing theforegoing description and/or the following claims.

The term “consisting essentially of” means that, in addition to therecited elements, what is claimed may also contain other elements(steps, structures, ingredients, components, etc.) that do not adverselyaffect the operability of what is claimed for its intended purpose asstated in this disclosure. Importantly, this term excludes such otherelements that adversely affect the operability of what is claimed forits intended purpose as stated in this disclosure, even if such otherelements might enhance the operability of what is claimed for some otherpurpose.

The term “individual”, “subject” or “patient” as used herein refers toany animal, including mammals, such as mice, rats, other rodents,rabbits, dogs, cats, swine, cattle, sheep, horses, or primates, andhumans. The term may specify male or female or both, or exclude male orfemale.

The terms “about” and “approximately” shall generally mean an acceptabledegree of error or variation for the quantity measured given the natureor precision of the measurements. Typical, exemplary degrees of error orvariation are within 20 percent (%), preferably within 10%, and morepreferably within 5% of a given value or range of values. For biologicalsystems, the term “about” refers to an acceptable standard deviation oferror, preferably not more than 2-fold of a given value. Numericalquantities given herein are approximate unless stated otherwise, meaningthat the term “about” or “approximately” can be inferred when notexpressly stated.

The term “nucleotides” as used herein refer to any such known groups,natural or synthetic. It includes conventional DNA or RNA bases (A, G,C, T, U), base analogs, e.g., inosine, 5-nitroindazole and others,imidazole-4-carboxamide, pyrimidine or purine derivatives, e.g.,modified pyrimidine base6H,8H-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one (sometimes designated“P” base that binds A or G) and modified purine baseN6-methoxy-2,6-diaminopurine (sometimes designated “K” base that binds Cor T), hypoxanthine, N-4-methyl deoxyguanosine,4-ethyl-2′-deoxycytidine, 4,6-difluorobenzimidazole and2,4-difluorobenzene nucleoside analogues, pyrene-functionalized LNAnucleoside analogues, deaza- or aza-modified purines and pyrimidines,pyrimidines with substituents at the 5 or 6 position and purines withsubstituents at the 2, 6 or 8 positions, 2-aminoadenine (nA),2-thiouracil (sU), 2-amino-6-methylaminopurine, O-6-methylguanine,4-thio-pyrimidines, 4-amino-pyrimidines,4-dimethylhydrazine-pyrimidines, O-4-alkyl-pyrimidines and hydrophobicnucleobases that form duplex DNA without hydrogen bonding. Nucleobasescan be joined together by a variety of linkages or conformations,including phosphodiester, phosphorothioate or methylphosphonatelinkages, peptide-nucleic acid linkages.

The term “polynucleotide” as used herein refers to a multimeric compoundcomprising nucleotides linked together to form a polymer, includingconventional RNA, DNA, LNA, BNA, copolymers of any of the foregoing, andanalogs thereof.

The term “nucleic acid” as used herein refers to a single strandedpolynucleotide or a duplex of two polynucleotides. Such duplexes neednot be annealed at all locations, and may contain gaps or overhangs.

The term “nick” as used herein refers to a discontinuity in a doublestranded nucleic acid molecule where there is no phosphodiester bondbetween adjacent nucleotides of one strand.

The term “miRNA” is used herein according to its ordinary and plainmeaning in the art, and refers to a microRNA molecule found ineukaryotes that is involved in RNA-based gene regulation. The term willbe used to refer to the single-stranded RNA molecule processed from aprecursor. Individual miRNAs have been identified and sequenced indifferent organisms, and they have been given names. The methods andcompositions should not be limited to miRNAs identified in theapplication, as they are provided as examples, not necessarily aslimitations of the invention.

Nucleic acids are “complementary” to each other, as used herein, when anucleotide sequence in one strand of a nucleic acid, due to orientationof its nucleotide hydrogen atoms, hydrogen bonds to another sequence onan opposing nucleic acid strand (of course, a strand of a nucleic acidmay be self-complementary as well). The complementary bases typicallyare, in DNA, A with T, and C with G, and, in RNA, C with G, and U withA. Complementarity can be perfect or substantial/sufficient. Perfectcomplementarity between two nucleic acids means that the two nucleicacids can form a duplex in which every base in the duplex is bonded to acomplementary base by Watson-Crick pairing. “Substantial” or“sufficient” complementary means that a sequence in one strand is notperfectly complementary to a sequence in an opposing strand, but thatsufficient bonding occurs between bases on the two strands to form astable hybrid complex at a given set of hybridization conditions (e.g.,salt concentration and temperature). Such conditions can be predicted byusing the sequences and standard models to predict the T_(m) ofhybridized strands, or by empirical determination of T_(m) by usingestablished methods. T_(m) refers to the temperature at which apopulation of hybridization complexes formed between two nucleic acidstrands are 50% denatured. At a temperature below the T_(m), formationof a hybridization complex is favored, whereas at a temperature abovethe Tm, melting or separation of the strands in the hybridizationcomplex is favored.

The term “ligase” as used herein refers to an enzyme that catalyzes theformation of a phosphodiester bond between two polynucleotides, orbetween the ends of a single polynucleotide. Ligases includeATP-dependent double-strand polynucleotide ligases, NAD+-dependentdouble-strand DNA or RNA ligases and single-strand polynucleotideligases. Specific examples of ligases include, but are not limited to,bacterial ligases such as E. coli DNA ligase and Taq DNA ligase,Ampligase® thermostable DNA ligase (Epicentre® Technologies Corp., partof Illumina®, Madison, Wis.), phage ligases such as T3 DNA ligase, T4DNA ligase and T7 DNA ligase and mutants thereof and T4 RNA ligase 1 andT4 RNA ligase 2 and mutants thereof such as Sso7 fusion proteins, T4truncated and mutated (K227Q) RNA ligase 2. In this disclosure the term“DNA/RNA ligase” or “RNA/DNA ligase” refers to a ligase that catalyzesthe formation of a phosphodiester bond between an RNA molecule and a DNAmolecule. Examples of DNA/RNA ligases include T4 DNA ligase and T4 RNAligase 2.

The term “ligative” means available for a ligation reaction, or asuitable substrate for a ligase.

B. Blocking Nucleic Acids

A blocking nucleic acid 120 for use in reducing the abundance of anon-target micro-RNA (miRNA) in an miRNA library is provided, including:a single-stranded complementary region 130 at one of the 5′ end of theblocking nucleic acid 120 or the 3′ end of the blocking nucleic acid120, that anneals with a binding region at a first end of the unwantedmiRNA 10; a hairpin loop forming region 140 or other double-strandedregion 170 adjacent to the complimentary region 130, in which all of theterminal ends of the blocking nucleic acid 120 except one areunavailable to participate in ligase reactions. The available terminalend will be immediately adjacent to the miRNA when the miRNA is annealedto the complementary region 130, leaving a nick that can be filed usingan appropriate ligase. The terminal ends are rendered unavailable toparticipate in ligase reactions by removing or masking the 4′ phosphategroup or 3′ hydroxyl group. In this disclosure, if the complementaryregion 130 of the blocking nucleic acid 120 is complementary to the 5′end of the unwanted miRNA 30 in question, it is referred to as a “5′blocking nucleic acid.” Similarly, if the complementary region 130 ofthe blocking nucleic acid is complementary to the 3′ end of the unwantedmiRNA 20 in question, it is referred to as a “3′ blocking nucleic acid.”

A first aspect of the blocking nucleic acid 120 comprises a hairpin loopforming region 140. Embodiments of the first aspect comprise asingle-stranded complementary region 130 at one of the 5′ end of theblocking nucleic acid 120 or the 3′ end of the blocking nucleic acid120, that anneals with a binding region at a first end of the unwantedmiRNA 10 under stringent conditions, wherein said first end is eitherthe 5′ end or the 3′ end of the unwanted miRNA 20, and wherein thecomplementary region 130 has a terminal end 132; a hairpin loop formingregion 140 adjacent to the complimentary region, the hairpin loopforming region 140 having a ligative terminal end 136; and a firstblocking moiety 180 linked to the terminal end of the hairpin loopforming region 140, in which said first blocking moiety 180 cannot serveas a substrate for ligases. The presence of the hairpin loop reduces thenumber of terminal ends that must be rendered unavailable for ligasereactions. It therefore has the advantage of simplifying the protocol.An embodiment of the first aspect of the blocking nucleic acid 120 isshown in FIG. 2 .

A second aspect of the blocking nucleic acid 120 does not necessary havea hairpin loop forming region 140, but has a double-stranded region 170that may be formed by a second strand (or by a hairpin loop or otherstructure of the first strand). Embodiments of the second aspect of theblocking nucleic acid 120 comprise: a Crick strand 220; a singlestranded complementary region 130 at one of the 5′ end of the Crickstrand 220 or the 3′ end of the Crick strand 220, that anneals with abinding region at a first end of the unwanted miRNA 10 under stringentconditions, wherein said first end is the 5′ end or the 3′ end of theunwanted miRNA (30 and 20, respectively); a double-stranded region 170on the Crick strand 220 adjacent to the complementary region 130, thedouble-stranded region 170 comprising a Watson strand 230 that isannealed to the Crick strand 220; a first blocking moiety 180 linked tothe 3′ end of the Crick strand 220, wherein the first blocking moiety180 cannot serve as a substrate for ligases; a second blocking moiety190 linked to the 5′ end of the Crick strand 220, wherein the secondblocking moiety 190 cannot serve as a substrate for ligases; a thirdblocking moiety 200 linked to the 3′ end of the Watson strand 230 if thecomplementary region 130 is at the 3′ end of the Crick strand 220, orlinked to the 5′ end of the Watson strand 230 if the complementaryregion 130 is at the 5′ end of the Crick strand 220, wherein the thirdblocking moiety 200 cannot serve as a substrate for ligases; and aligative terminal end 210 on the Watson strand 230, the ligativeterminal end 210 located at the 3′ end of the Watson strand 230 if thecomplementary region 130 is at the 5′ end of the Crick strand 220, or atthe 5′ end of the Watson strand 230 if the complementary region 130 isat the 3′ end of the Crick strand 220. An embodiment of the secondaspect of the blocking nucleic acid 120 is shown in FIG. 22 .

The miRNA is referred to as “unwanted,” as one useful application of theblocking nucleic acid 120 is to reduce the abundance of over-representedmiRNAs in miRNA libraries, but the blocking nucleic acid 120 can be usedto bind to one end of any RNA molecule for a variety of applications.The descriptor “unwanted” should not be seen as an indication that theblocking nucleic acid 120 cannot or should not be used with any givenRNA or type of RNA.

The complementary region 130 is described as single-stranded as it mustbe non-annealed in order to anneal with the miRNA, which is critical toits functioning. Of course, the blocking nucleic acid 120 could beprepared such that the complementary region 130 is annealed with anotherpolynucleotide prior to use (for example, to aid in stability duringstorage and prevent dimerization), and then denatured in preparation foruse. The complementary region 130 will be designed to be of sufficientlength to be specific to its intended target(s), but short enough tobind easily to the binding region at the annealing temperature. Someembodiments of the complementary region 130 are about 5-50 nucleotidesin length. Further embodiments of the complementary region 130 are about8-20 nucleotides in length. Still further embodiments of thecomplementary region 130 are about 10-15 nucleotides in length. In aspecific embodiment of the blocking nucleic acid 120, the complementaryregion 130 is 12 nucleotides in length.

The complementary region 130 is described as being at the 5′ end or the3′ end of its associated polynucleotide to assure that the respectiveterminal end 210 is available for ligation. It is possible, however,that a blocking polynucleotide could be designed to place thecomplementary region 130 proximate to the 3′ or 5′ end, but not at theterminal end 210 itself; in such an embodiment, after annealing with theunwanted miRNA 10 the un-annealed tail could be clipped off with anendonuclease. After such endonuclease removal, and prior to ligation,the complementary region 130 would in fact be at the 5′ or 3′ end of thepolynucleotide.

The complementary region 130 can be designed to anneal with a knownsequence at the 3′ or 5′ end of an miRNA by those skilled in the artwithout undue experimentation. Thousands of miRNAs are known, and theirsequences can be searched using online resources such as PHENOMIR 2.0(provided by the Helmholtz Zentrum Munchen—German Research Center forEnvironmental Health IBIS Institute of Bioinformatics and SystemsBiology, and available at mips.helmholtz-muenchen.de) and MIRBASE.org,managed by the Griffiths-Jones lab at the Faculty of Life Sciences,University of Manchester.

For example, one of the commonly over represented miRNA 10 molecules inmiRNA sequencing libraries is mir-16, a miRNA that has been implicatedin the development of B-cell lymphocytic leukemia in addition to breast,colon, brain, lung, prostate and stomach cancers. mir-16 is expressed inmany tissue types and is often over-represented in miRNA sequencinglibraries. Accordingly, the ability to decrease the overall frequency ofmir-16 in sequencing libraries would be beneficial. Other overrepresented or abundant miRNAs include, but are not limited to mir-486,mir-451a and mir-26. Table 1 shows the nucleotide sequences of severalblocking nucleic acids (DNAs) 120 and their respective target miRNAs 10.The complementary regions 130 of each are shown in white type on blackbackground. The stem-and-loop forming regions of each are shownunderlined. Note the consensus sequence between all four blockingnucleic acids in Table 1 at positions 13-58 (SEQ ID NO: 5). Note alsothat all of SEQ ID NO. 1-4 have stem-and-loop forming regions, and soform blocking nucleic acids having only one strand. It should be notedthat in some situations a single blocking nucleic acid 120 mayeffectively reduce the frequency of more than one unwanted, overrepresented, or abundant miRNA 10 simultaneously.

TABLE 1 Exemplary 5′ Blocking Nucleic Acids SEQ  miRNANucleotide Sequence (5′ to 3′) ID NO. mir-486

1 mir-26

2 mir-451

3 mir-16

4

It is contemplated that the complementary region 130 of any of the 5′blocking nucleic acids 120 will share a certain level of identity withpositions 1-12 of one of SEQ ID NO: 1-4. The certain level of identitymay be selected from at least 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%,99%, and 100%. In a further embodiment, the certain level of identity isgreater than 95%. In still further specific embodiments, the blockingnucleotide comprises one of SEQ ID NO: 1-4.

Some embodiments of 3′ nucleic acid blockers specific to hsa-miR-16-5pcomprise a sequence with a certain level of identity with SEQ ID NO: 13.The certain level of identity may be selected from at least 50%, 60%,70%, 75%, 80%, 85%, 90%, 95%, 99%, and 100%. In a further embodiment,the certain level of identity is greater than 95%. The 3′ end of SEQ IDNO: 13 is the complementary region 130 to a binding region on the 3′ endof hsa-miR-16-5p.

The complementary region 130 will anneal with a binding region at thefirst end of the unwanted miRNA 10 under stringent conditions. Suchstringency is based on the melting temperature (T_(m)) of the nucleicacid binding complex, as taught in Berger and Kimmel (1987, Guide toMolecular Cloning Techniques, Methods in Enzymology, 152, AcademicPress, San Diego Calif.). The T_(m) of an annealed duplex depends on thebase composition of the duplex, the frequency of base mismatches, andthe ionic strength of the reaction medium. The T_(m) of a duplex can becalculated by those of ordinary skill in the art based on these twofactors using accepted algorithms. Maximum stringency typically occursat about 5° C. below T_(m); high stringency at about 5-10° C. belowT_(m); intermediate stringency at about 10-20° C. below T_(m); and lowstringency at about 20-25° C. below T_(m). As will be understood bythose of skill in the art, a maximum stringency hybridization can beused to identify or detect identical nucleotide sequences while anintermediate (or low) stringency hybridization can be used to identifyor detect similar or related sequences. The term “stringent” by itselfin this context refers to intermediate stringency. Terms such asmaximally stringent, highly stringent, and poorly stringent, refer toconditions of maximal stringency, high stringency, and low stringencyrespectively.

An example of maximally stringent conditions is provided in the workingexample below. Specifically, the stringent conditions may be theconditions set forth in the “Supplemental Methods” section of WorkingExample 1, under “Blocking Ligation.” Note that the hybridizationtemperature in that example was 30° C., while the calculated T_(m) ofthe duplex between the blocking nucleic acid 120 and the miRNA was 35°C.

The complementary region 130 will generally function to anneal understringent conditions with at least 5 consecutive bases in the miRNA.Examples of the sequences of overrepresented miRNAs in miRNA librariesare provided in SEQ ID NO: 6-11. In embodiments of the blocking nucleicacid 120 useful to block those miRNAs, the complementary region 130 mayanneal under stringent conditions with at least 5 consecutive bases ofat least one of SEQ ID NOS: 6-11. In some such embodiments, thecomplementary region 130 may anneal under stringent conditions with atleast 8 consecutive bases of at least one of SEQ ID NOS: 6-11. Infurther embodiments, the complementary region 130 may anneal understringent conditions with at least 10 consecutive bases of at least oneof SEQ ID NOS: 6-11. In further embodiments, the complementary region130 may anneal under stringent conditions with positions 2-8 of at leastone of SEQ ID NOS: 6-11 (SEQ ID NO: 12). In specific embodiments, thecomplementary region 130 may anneal under stringent conditions withpositions 1-9 of at least one of SEQ ID NOS: 6-11.

In further embodiments of the blocking nucleic acid 120, thecomplementary region 130 will anneal with a binding region at the firstend of the unwanted miRNA 10 under highly stringent conditions. In stillfurther embodiments of the blocking nucleic acid 120, the complementaryregion 130 will anneal with a binding region at the first end of theunwanted miRNA 10 under maximally stringent conditions.

The blocking moieties are moieties that are not available for ligationreactions, i.e., they cannot serve as substrates for ligases. Variousknown ligases are capable of ligating specific nucleic acids, but notothers. Ligases all require the nucleotides to be ligated have anavailable 3′ hydroxyl group and an available 5′ phosphate group. Someembodiments of the blocking moieties are nucleotides from which the 3′hydroxyl group has been removed or the 5′ phosphate group has beenremoved (or possibly both). The blocking moieties could also benon-nucleotide groups bonded to the terminal nucleotide in the strand.Such non-nucleotide groups include “spacers” such as C3 spacer(phosphoramidite), Spacer 9 (triethylene glycol), and Spacer 18(hexa-ethyleneglycol). Other non-nucleotide spacers can include a propylgroup, a propanol group, other organic alcohols, and other glycolcompounds. Examples of nucleotide blocking moieties include an inverteddeoxynucleotide, a dideoxynucleotide, and an inverted dideoxynucleotide.The first, second, and third blocking moieties when present may be thesame moieties, or they may be independently selected, so long as eacheffectively prevents the associated polynucleotide from undergoingligation.

The blocking moiety may be linked directly or indirectly to the blockingnucleic acid 120. If linked indirectly, a linker group may be presentbetween the blocking group and the terminal nucleotide. Such linkergroups may include, for example, Spacer 9 (triethylene glycol) andSpacer 18 (hexa-ethyleneglycol).

In contrast, the blocking nucleic acid 120 also has a ligative terminalend (136 or 210). In the first aspect (hairpin loop) of the nucleicacid, the ligative terminal end 136 is found at the end of thehairpin-loop forming region 140. In the second aspect, the ligativeterminal end 210 is found at one end of the Watson strand 230. Theligative terminal end (136 or 210) is intended to be ligated to one ofthe ends of the miRNA. The ligative terminal end (136 or 210) will inmany embodiments be a terminal nucleotide with an available 3′ hydroxylgroup, an available 5′ phosphate group, or both. In some embodiments ofthe blocking nucleic acid 120, the ligative terminal end (136 or 210) isa natural nucleotide (e.g., A, T, C, G, U) with an available 3′ hydroxylgroup, an available 5′ phosphate group, or both. In further embodiments,the ligative terminal end (136 or 210) is a non-natural nucleotide withan available 3′ hydroxyl group, an available 5′ phosphate group, orboth. A group is “available” if it has at least one oxygen atom that canform a phosphodiester bond, and is not sterically hindered (or otherwisehindered) from doing so.

Some embodiments of the blocking nucleic acid 120 comprise the hairpinforming region. The presence of the hairpin forming region reducing thenumber of terminal ends that require blocking to avoid unwantedligation. A hairpin loop occurs when two regions of the same strand,usually complementary in nucleotide sequence when read in oppositedirections, base-pair to form a double helix that ends in an unpairedloop. The formation of a stem-loop structure is dependent on thestability of the resulting helix and loop regions. The firstprerequisite is the presence of a sequence that can fold back on itselfto form a paired double helix. The stability of this helix is determinedby its length, the number of mismatches or bulges it contains (a smallnumber are tolerable, especially in a long helix) and the basecomposition of the paired region. The stability of the loop alsoinfluences the formation of the stem-loop structure. Loops that are lessthan three bases long are sterically impossible and do not form. Largeloops with no secondary structure of their own (such as pseudoknotpairing) are also unstable. Optimal loop length tends to be about 4-8bases long. Commonly used 4 base pair loops (“tetraloops”) include ANYA,CUYG, GNRA, UMAC and UNCG. Suitable hairpin loop structures can bedesigned by those of ordinary skill in the art. Specific embodiments ofthe hairpin loop forming region 140 comprise a sequence with a certainlevel of identity with SEQ ID NO: 5. The certain level of identity maybe selected from at least 50%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, 99%,and 100%. In a further embodiment, the certain level of identity isgreater than 95%.

In the second aspect of the nucleic acid blocker, a double strandedregion is present that is not necessarily a hairpin loop formingstructure. It is described as having a Watson strand 230 and a Crickstrand 220, although in some cases this may be the same strand foldedover on itself. The terms “Watson” and “Crick” have no descriptive orrestrictive meaning, except to mean that the two strands are at leastpartially annealed to one another. In the absence of a hairpin loop,blocking moieties as described above are present on both ends of theCrick strand 220. A blocking moiety will also be present on the end ofthe Watson strand 230 farthest from the single-stranded region. In thisparticular context, the blocking moieties on the two terminal ends (onWatson and Crick) farthest from the single-stranded region may beembodied in a polynucleotide linking Watson and Crick. As describedabove, a hairpin loop can serve this function, but the polynucleotidelinking Watson and Crick need not have a hairpin loop structure to servethe purpose of making the two terminal ends farthest from thesingle-stranded complementary region 130 unavailable for ligation.

In the second aspect of the blocking nucleotide, the Watson strand 230has only one blocking moiety, referred to as the “third blocking moiety”(200). As shown in FIG. 22 , the third blocking moiety 200 will be onthe terminal end of Watson farthest from the single-strandedcomplementary region 130. As the terminal end of Watson that is closestto the complementary region 130 must be ligated with the miRNA, it willnot be blocked.

C. Methods of Excluding miRNA from RT-PCR

A method of preventing the unwanted miRNA 10 from participating inRT-PCR is provided. In a general embodiment, the method comprisesannealing any of the blocking nucleic acids 120 described above to thebinding region of the unwanted miRNA 160. After annealing, the blockingnucleic acid 120 may be ligated to the “first end” of the miRNA wherethe binding region 160 is located. As should be apparent from theschemes shown in FIGS. 2 and 22 , the binding region 160 will be on the5′ end of the miRNA if the complementary region 130 is at the 5′ of theblocking nucleic acid 120, and the binding region 160 will be on the 3′end of the miRNA if the complementary region 130 is at the 3′ end of theblocking nucleic acid 120.

The annealing step will generally be carried out under at leastintermediate stringent conditions. The higher the stringency, the lesslikely it will be that the complementary region 130 binds to anunintended miRNA. In some embodiments of the method, the annealing stepwill be carried out under highly stringent conditions or maximallystringent conditions.

The ligating step is carried out using the appropriate ligase. If theblocking nucleic acid 120 is an RNA, then an RNA/RNA ligase must beused. If the blocking nucleic acid 120 is a DNA, then a DNA/RNA ligasemust be used. Many ligases of both types are commercially available, andtheir properties and protocols for their use are known to those ofordinary skill in the art. If the blocking nucleic acid 120 is a DNA,then the ligase may be for example T4 DNA ligase, which ligates RNA toDNA when an RNA/DNA duplex has been formed.

The method may be carried out on any RNA, and as explained above theterm “unwanted” to characterize the miRNA refers only to one intendeduse of the method, and does not limit the structure or source of the RNAinvolved. Some embodiments of the method are for the purpose ofpreventing one or more unwanted miRNAs 10 from participating in RT-PCR,and in such embodiments the miRNA may be “unwanted” because it is a veryabundant or overrepresented miRNA in a sample. Examples of such abundantmiRNA include mir-16, mir-486, mir-451, and mir-26. Accordingly, in someembodiments of the method the unwanted miRNA 10 is selected from thosemiRNAs. In some cases the complementary region 130 and annealingconditions may be designed to allow the complementary region 130 toanneal with more than one miRNA, and any such additional miRNAs could beany taught to be suitable in the method by themselves.

The product of the method will be an miRNA that is annealed to theblocking polynucleotide (“blocked miRNA complex” 500). Such a blockedmiRNA complex 500 will be unable to participate in at least one of a 5′ligation reaction or a 3′ ligation reaction.

D. Reducing the Abundance of Unwanted miRNA in an miRNA Library

A method of reducing the abundance of an unwanted miRNA 10 in an miRNAlibrary is provided, using any of the blocking nucleic acids 120provided above. A general embodiment of the method comprises thefollowing steps in no particular order: (a) purifying RNA from a samplecomprising a plurality of miRNAs; (b) introducing an adenylated nucleicacid adapter 40 and a first DNA/RNA ligase 50 under conditions to allowthe adenylated nucleic acid adapter 40 to ligate to the 3′ end of theplurality of miRNAs; (c) introducing any of the blocking nucleic acids120 disclosed above under conditions to allow the complementary region130 of the blocking nucleic acid 120 to anneal the binding region of theunwanted miRNA 160, to produce a blocked sample 450; (d) introducing anRNA adapter 70 and an RNA ligase 80 under conditions to allow the RNAadapter 70 to ligate the 5′ end of the plurality of miRNAs; (e)introducing a reverse transcriptase to the blocked sample 450 underconditions to allow reverse transcription of the plurality of miRNAs, toproduce a cDNA sample; and (f) performing PCR on the cDNA sample toproduce the miRNA library with reduced abundance of the unwanted miRNA10.

Steps such as the purification of RNA from a biological sample, ligatingadapters to the 3′ end and 5′ end of the miRNA, in vitro reversetranscription, and PCR can be performed according to any suitableprotocol known in the art. Some exemplary protocols can be found in theTruSeq®RNA Access Library Prep Guide, Illumina, Inc., San Diego, Calif.(2014).

Some embodiments of the method comprise introducing a second DNA/RNAligase 150 under conditions to allow the blocking nucleic acid 120 toligate to one of the 5′ end and the 3′ end of the unwanted miRNA (30 and20, respectively). As is apparent from FIGS. 2 and 22 , the blockingnucleic acid 120 will ligate to the end of the miRNA where the bindingregion 160 is located.

The adenylated nucleic acid adapter 40 may be any type of nucleic acid,including but not limited to DNA or RNA. Adenylated DNA adapters 40 havethe advantage of superior stability, and are not vulnerable toubiquitous RNAses. Some embodiments of the adenylated nucleic acidadapter 40 comprise a reverse transcriptase primer binding site. As isknown in the art, reverse transcriptase enzymes require the binding of aprimer before reverse transcribing RNA. Most known reverse transcriptaseenzymes use tRNAs as primers. In retroviruses, plant pararetroviruses,and transposons containing long terminal repeats, reverse transcriptionis primed by specific tRNAs. All these retroelements contain a primerbinding site complementary to the primer tRNA. The tRNAs most widelyused as primers are tRNA(Trp), tRNA(Pro), tRNA(1,2Lys), tRNA(3Lys),tRNA(iMet). Other tRNAs such as tRNA(Gln), tRNA(Leu), tRNA(Ser),tRNA(Asn) and tRNA(Arg) are also occasionally used as primers. In theretroviruses and plant pararetroviruses, the primer binding site iscomplementary to the 3′ end of the primer tRNA. In the case ofretrotransposons, the primer binding site is either complementary to the3′ end or to an internal region of the primer tRNA. Those of ordinaryskill in the art will select the reverse transcriptase primer bindingsite on the adenylated nucleic acid adapter 40 to bind whichever primeris known to function with the reverse transcriptase that has beenselected for the method.

If a 5′ blocking nucleic acid 120 is used, the adenylated nucleic acidadapter 40 may be ligated to the miRNA before or after annealing it tothe blocking nucleic acid 120. In this situation the adenylated nucleicacid adapter 40 will often be ligated to the 3′ end of the miRNA beforeligating the blocking nucleic acid 120, to avoid the need to reduceresidual ATP from the previous step. When using a 5′ blocking nucleicacid 120, the step in which the blocking nucleic acid 120 is ligated tothe miRNA will precede ligation of the RNA adapter 70 to the 5′ end ofthe miRNA; otherwise blocking would be ineffective.

If a 3′ blocking nucleic acid 120 is used, the adenylated nucleic acidadapter 40 must be ligated to the 3′ end of the miRNA after annealing itto the blocking nucleic acid 120, or else blocking would be ineffective.In such cases it may be desirable to remove excess ATP left over fromthe ligation of the blocking nucleic acid 120 prior to ligating theadenylated nucleic acid adapter 40 to the 3′ end of the miRNA. This canbe done by any of several methods. For example, the sample may be runthrough a chromatographic column after ligating the blocking nucleicacid 120 to the 3′ end of the miRNA, and prior to ligating theadenylated nucleic acid adapter 40 to the 3′ end of the miRNA. Theexcess ATP could also be removed by chemical reaction, electrophoresis,or other methods. When a 3′ blocking nucleic acid 120 is used, ligationof the RNA adapter 70 to the 5′ end of the miRNA may occur before orafter ligation of the blocking nucleic acid 120 to the 3′ end of themiRNA.

The steps of reverse transcribing the blocked sample 450 and PCRamplifying the cDNA sample will occur in that order, and will occurafter the steps marked (a)-(d) above.

The annealing will be carried out under stringent conditions, asdescribed in previous sections of this disclosure. In some embodimentsof the method, annealing will be carried out under highly stringentconditions or maximally stringent conditions. The creation of a duplexbetween the blocking nucleotide and the miRNA will leave a nick betweenthem that will be linked by the DNA/RNA ligase.

The first DNA/RNA ligase 50 will be a ligase capable of ligating theadenylated nucleic acid adapter 40 to an RNA. One example is T4 RNAligase 2, truncated. The truncated version has the desirable property ofrequiring that the DNA have an adenylated 5′ terminal end, and so willfunction to specifically bind an adenylated nucleic acid adapter 40 tothe 3′ end of the miRNA. In order to prevent unwanted ligation products,the adenylated nucleic acid adapter 40 may have a blocking moiety at its3′ end. The blocking moiety may be any that is described above assuitable for use in the blocking nucleic acid 120.

The second DNA/RNA ligase 150 will be a ligase capable of ligating thebinding region 160 of the miRNA to the double-stranded region 170 of theblocking nucleic acid 120. As such, it is preferably able to ligate anick in a duplex between a DNA polynucleotide and an RNA polynucleotidewhere an overhang exists. A specific example of such a DNA/RNA ligase isT4 DNA ligase.

The RNA ligase 80 will be a ligase capable of ligating the 5′ end of themiRNA to the 3′ end of the RNA adapter 70. Any such ligases known in theart may be used. In some embodiments of the method the RNA ligase 80will be T4 RNA ligase 1, which is capable of ligating single-strandedRNA and DNA as well as dinucleoside pyrophosphates.

The RNA adapter 70 serves to provide a known primer binding site duringPCR. Consequently, it may correspond in length to a suitable length fora primer binding site. Various embodiments of the RNA adapter 70 mayhave lengths selected from 5-30 bases and 18-22 bases.

The miRNA library with reduced abundance of the unwanted miRNA 10 thatis a product of the method is also provided. In this context “reducedabundance” refers to there being significantly less of the unwantedmiRNA in the miRNA library than would be observed in a library of thesame sample or a similar sample in which no blocker is used. FIG. 14-20are clear illustrations of such reduced abundance. In some embodimentsof the miRNA library, the abundance of the unwanted miRNA has beenreduced by at least 50%. In further embodiment of the miRNA library, theabundance of unwanted miRNA has been reduced at least by an amountselected from the group consisting of: 60%, 70%, 75%, 80%, 85%, 90%,95%, 97.5%, 98%, 99%, 99.5%, 99%, and 100%. Put another way, the reducedabundance may be at least a 2-fold reduction in abundance. In someembodiments of the method, the reduced abundance may be at least a4-fold reduction, at least a 6-fold reduction, or at least an 8-foldreduction.

E. Kits

Kits are provided for reducing the frequency of an miRNA in an miRNAlibrary, comprising any of the blocking nucleic acids 120 disclosedabove. The kit may further comprise the reagents, buffers, enzymes,instruction booklets, positive and negative controls and other materialsuseful or necessary to carry out the methods described herein. Suchadditional materials may include those useful to RNA/RNA ligation,RNA/DNA ligation, PCR, reverse transcription, in situ hybridization, andRNA purification. Specific non-limiting examples of additional kitcomponents include a container of any of the following: DNA/RNA ligase610 capable of ligating DNA to RNA when annealed (for example, T4 DNAligase 620); RNA/RNA ligase 630 (for example, T4 RNA ligase 1 (640)); anRNA/DNA ligase 650 (for example, T4 RNA ligase 2, truncated (660)); aplurality of DNA primers 680; a nucleotide solution 690; a PCR buffer700; a thermophilic DNA polymerase 710, an adenylated nucleic acidadapter 720, and an RNA adapter 730. The listed additional kitcomponents may be any that are described as suitable for the methodsabove. A “container of” the listed component may be any sort ofcontainer as could be easily designed by those of ordinary skill in theart. The container may contain more than one listed component, or it maycontain other components apart from the ones listed. In some embodimentsof the kit, the container contains only the listed component to theexclusion of others (although not necessarily to the exclusion ofinactive substances such as buffers, solvents, etc.). As a result,reference to a kit comprising “a container of X and a container of Y”should be read to encompass a kit comprising two separate containerscontaining X and y respectively, and a kit comprising one containercontaining X and Y. In some embodiments of the kit, a given componentmay have its own container.

F. Nucleic Acids

Nucleic acid molecules are provided for use in the blocking nucleicacids 120, methods, and kits above. These include a nucleic acidcomprising a sequence having at least a certain level of identity withany one of SEQ ID NO: 1-4 and 13. The level of identity may be ≥50%,60%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, and 100%. In a specificembodiment the level of identity is >95%. The nucleic acids also includethose that anneal under stringent conditions with any of the foregoing.Some embodiments of the nucleic acid anneal to any of the foregoingunder highly stringent conditions. In some further embodiments of thenucleic acid, the nucleic acid anneals to any of the foregoing undermaximally stringent conditions. In a specific embodiment of the nucleicacid, the nucleic acid is the exact complement to any of the foregoingnucleic acids. These molecules may be any type of nucleic acid,including RNA, DNA, LNA, BNA, copolymers of any of the foregoing, andanalogs thereof. In a specific embodiment, the nucleic acid is DNA.

A blocked miRNA complex 500 is also provided, comprising any of theblocking nucleic acids 120 disclosed above, and the unwanted miRNA 10annealed to the complementary region 130. In some embodiments of theblocked miRNA complex, the blocking nucleic acid 120 is ligated to thefirst end of the miRNA. Some embodiments of the blocked miRNA complex500 are the product of any of the methods of preventing an unwantedmicro-RNA (miRNA) from participating in RT-PCR provided above. As suchmethods may be used to prevent RT-PCR on very abundant oroverrepresented miRNAs in a sample, the miRNA may be any of mir-16,mir-15a, mir-15b, mir-195, mir-424, mir-497, mir-486, mir-451, andmir-26. The miRNA may comprise a sequence selected from SEQ ID NO: 6(miR-16), 7 (miR-15a), 8 (miR-15b), 9 (miR-195), 10 (miR-424), 11(miR-497), and 12 (consensus positions 2-8 of the foregoing). Organismsand vectors comprising any of the nucleic acids above are also provided.Examples of uses for such organisms and vectors are production of thenucleic acids, cloning of the nucleic acids, and stable storage of thesame. Many suitable vectors are known in the art, such as viruses,plasmids, cosmids, fosmids, phagmids, artificial chromosomes, yeastartificial chromosomes, human artificial chormosomes, planttransformation vectors, and liposomes. Unicellular organisms areparticularly useful in cloning, replicating, and maintaining nucleicacids of interest. Model unicellular organisms that are commonly usedfor this purpose include yeasts, other fungi, bacteria, protists, andarchaea.

Specific model organisms are well known in the art, and include bacteriasuch as Escherichia coli, Salmonella typhimurium, Pseudomonasfluorescens, Bacillus subtilis, Mycoplasma genitalium, and variousSynechocystis sp.; protists such as Dictyostelium discoideum,Tetrahymena thermophila, Emiliania huxleyi, and Thalassiosirapseudonana; and fungi such as Aspergillus sp., Neurospora crassa,Saccharomyces cerevisiae, and Schizosaccharomyces pombe.

G. Examples I. Working Example #1: Ligation of Blocking Nucleic Acids to5′ End of miRNA

Materials and Methods

Total RNA Isolation

The protocol for collection of peripheral blood samples was approved bythe Institutional Review Board at the University of Alabama atBirmingham, and all donors provided written, informed consent. Blood wascollected into EDTA tubes. Within 30 min of collection, the plasma wasisolated (˜5 ml) and stored at −80° C. One milliliter plasma wascentrifuged at 14,000 relative centrifugal force for 15 min and totalRNA was isolated from the supernatant using the Plasma/Serum Circulatingand Exosomal RNA Purification Kit (Slurry Format) (Norgen Biotek)following the manufacturer's directions. The eluate from this kit wasfurther concentrated using the RNA Clean-Up and Concentration Kit(Norgen Biotek) using 20 μl elution buffer to collect the RNA.

Small RNA Sequencing and miRNA Blocking

Isolated total RNA containing miRNA was converted to cDNA sequencinglibraries according to the method described in Vigneault et al. (2012)and Eminaga et al. (2013), with modification (the full protocol can befound in Supplemental Methods). Briefly, for each library, 4 μl isolatedRNA was combined with one μl of 10 μl 3′ adaptor and 1 μl T4 RNA Ligase2, truncated (NEB) in the appropriate buffer for 1 h. Simultaneously, 1μl 0.5 μM miRNA blocking oligonucleotide was incubated for 5 min at eachof the following temperatures: 95° C., 65° C., 55° C., 45° C. and 35° C.to ensure the proper formation of the hairpin structure. Next, incubatedblocking oligonucleotide was added to the 3′ adaptor ligation productand incubated for 1 h at 30° C. and 15 min at 65° C. in the presence ofT4 DNA Ligase (NEB) in the appropriate buffer to anneal and block thetargeted miRNA from further reactions. One microliter of 10 μM reversetranscription primer was annealed to the 3′ adaptor ligation product for5 min at 75° C., 30 min at 37° C. and 15 min at 25° C. prior to theaddition of the 5′ adaptor in order to reduce formation of adaptor-dimerproducts. One microliter of 20 μM pooled 5′ adaptor was incubated for 2min at 70° C. and then ligated with T4 RNA Ligase 1 (NEB) to eachreaction product for 1 h at 25° C. Ligated reaction products werereverse transcribed using SuperScript II (Invitrogen) and amplified viaPCR using Phusion High-Fidelity PCR Master Mix (NEB). The thermalcycling conditions were 94° C. for 30 s, followed by 15 cycles of 94° C.for 10 s and 72° C. for 45 s and a final extension at 65° C. for 5 min.

Libraries were cleaned and concentrated using a MinElute PCRPurification Kit (Qiagen), following the manufacturer's instructions,and eluted into a final volume of 20 μl. Libraries were separated on aTBE-Urea 10% acrylamide gel (Bio-Rad) with warm buffer for 50 min. Theband corresponding to miRNAs (˜135-145 base pairs) was excised, elutedfrom the gel, precipitated and resuspended in 10 μl of EB Buffer(Qiagen). Small RNA library concentration was quantified by the LibraryQuantification Kit—Illumina/ABI Prism (KAPA Biosystems) and sequenced ona HiSeq2000 or aMiSeq according to standard Illumina protocols.

Data Processing and Analysis

Adaptor sequences were trimmed from the raw fastq files using Cutadapt(37). The trimmed reads were aligned to pre-miRNA sequences (miRBaseversion 19) (38) using Bowtie2 (39). The alignments were filtered tokeep only those alignments that had two or fewer base mismatches andyielded a unique best alignment as measured by the Bowtie2 alignmentscore. The remaining unaligned reads were then aligned to the hg19reference genome using Bowtie2. Again, unique best reads were required.For miRNAs, read counts were obtained by counting the overlaps of thereads aligned to the pre-miRNAs with the canonical mature formboundaries (miRBase version 19) using BEDtools (40). Any overlap withthe mature region was counted. The miRNA read counts for each experimentwere down-sampled to a common level using random sampling implemented inR (base package). When the ‘average’ of two replicates was taken(generally for plotting), the following procedure was used: the twolibraries were down-sampled to a common total count value and thencounts for each species were summed. This summed library was thendown-sampled to the original common total count value. This processes isfavored for averaging replicate libraries because it preserves the countnature of the data and accordingly the underlying distribution.Differential expression was calculated using the package DESeq2 (41) inR using ‘local’ dispersion estimates and ‘LRT’ tests. A significantresult was defined as one with Benjamini-Hochberg adjusted P-value<0.01. Dispersion estimates were calculated with DESeq2 as well usingthe ‘local’ mode. Prior to plotting in FIG. 6A, the estimates weresmoothed using the spline function in R (base package).

Results

Blocking Hsa-miR-16-5p in Sequencing Libraries

In a set of small RNA sequencing libraries from 27 human plasma samplesthat were prepared by using a slightly modified version of the protocoldescribed by Alon et al. (28), reads mapping to hsa-miR-16-5p comprisedbetween 20 and 60% of the total aligned reads in the libraries (FIG. 8). Furthermore, consistent with other reports (32, 33), hsa-miR-16-5plevels correlated with the degree of hemolysis present in the sample.The massive abundance of hsa-miR-16-5p in these libraries makessequencing to a sufficient depth to detect lowly abundant miRNAs veryexpensive. Proper normalization of libraries in which one or few speciesdominate the reads is problematic. Also, because the hsa-miR-16-5p levelvaries, sequencing multiple samples to a common depth, in terms ofnon-hsa-miR-16-5p reads, is difficult.

To resolve these issues, an approach was devised to remove hsa-miR-16-5pfrom the sequencing libraries by blocking it as a substrate of T4 RNALigase 1 during the ligation of the adaptor to the 5′ end (FIG. 2 ). Inthe standard protocol (FIG. 1 ), a pre-adenylated DNA oligonucleotideadaptor is ligated to the 3′ ends of the pool of small RNA species usingtruncated T4 RNA Ligase 2. Subsequently, a RNA oligonucleotide adaptoris ligated to the 5′ ends using unmodified T4 RNA Ligase 1. Theresulting product is reverse transcribed and amplified with PCR. In themodified protocol (FIG. 2 ), use was made of an oligonucleotidecomprised of a self-complementary hairpin with a 12-base overhang on its5′ end that is the reverse complement of the first 12 bases of the 5′end of the canonical sequence of the targeted miRNA. The 5′ end of theoligonucleotide is modified with a C3 spacer (propyl group) to prohibitits participation in any unwanted ligation reactions. This ‘blocker’oligonucleotide is introduced after the ligation of the pre-adenylatedadaptor to the 3′ ends of the small RNA pool but prior to the ligationof the adaptor to the 5′ ends. The complementary portions of thetargeted miRNA species and the blocker participate in Watson-Crick basepairing to form a double stranded RNA:DNA hybrid with a missingphosphodiester bond between the 3′ end of the blocker and the 5′ end ofthe targeted miRNA, comprising a ‘nick’. T4 DNA Ligase recognizes thishybrid molecule and seals the nick (NEB product literature), resultingin the blocker being covalently bound to the 5′ end of the target miRNA.The presence of the hairpin and the C3 blocker prevent the subsequentligation of the adaptor to the 5′ end of this product. Without theprimer binding sequence contained in the adaptor, this ‘blocked’ productis not amplified in downstream PCR, effectively removing it from thefinal library.

To demonstrate the efficacy of this approach, various concentrations ofa blocker targeting hsa-miR-16-5p were titrated into library generationreactions using human heart total RNA as the input. Human heart totalRNA is a suitable test sample since hsa-miR-16-5p is abundant inlibraries derived from it, comprising ˜10% of the miRNA reads. Theeffect on hsa-miR-16-5p read abundances in the final sequenced librariesshows dose-response behavior (FIG. 3 ), with a maximal effect in the5-20 nM range. Furthermore, this blocking method was applied by usingthe hsa-miR-16-5p blocking oligonucleotide at 20 nM in a set oflibraries derived from 23 human plasma samples. In the sequencedlibraries, hsa-miR-16-5p was reduced to <1% of the reads in all cases(FIG. 9 ), far lower than in the previous set without blocking (FIG. 8).

Because it was anticipated that targeting miRNAs at their 5′ ends wouldlead to off-target activity due to sequence homology within miRNAfamilies, it was initially attempted to target and block miRNAs from the3′ end. Analogous to the 5′ approach, a hairpin oligonucleotide was usedwith a complementary 3′ overhang, a 5′ phosphate and 3′ C3 blocker. Theblocking ligation with T4 DNA Ligase occurs first, before the ligationof the adaptor to the 3′ ends of the small RNA pool. Although thisapproach did effectively block hsa-miR-16-5p in human heart total RNA(data not shown), it had an adverse effect on the final library yields.In fact, even in libraries subjected to a mock blocking ligationreaction that included all reagents except the blocker oligonucleotide,this 3′ approach yielded final library concentrations approximately fivetimes lower than the 5′ approach (FIG. 10 ). This decrease in yield inthe 3′ approach is likely due to the leftover ATP from the initialblocking ligation with T4 DNA Ligase inhibiting the truncated T4 RNALigase 2 in the subsequent ligation of the adaptor to the 3′ ends of thesmall RNA pool. Although truncated T4 RNA Ligase 2 cannot turnover ATP,ATP can still bind to the remnants of the active site, leading toinhibition of the enzyme (personal communication with NEB). Thus, a 3′approach could likely be implemented without unwanted consequences ifthe reaction components of the blocking ligation were removed via columnpurification or some other suitable method. However, the fractionalrecovery of the small RNA from these methods can be low. Considering theintended application of this method to human plasma samples in which theRNA concentrations are already low, further reduction of the effectiveRNA input is undesirable. Furthermore, miRNAs are known to haveconsiderable variation at their 3′ ends due to differences in Dicer cutsites and non-templated nucleotide additions (42, 43). Because theapproach relies on T4 DNA Ligase, which is sensitive to base-pairmismatches and gaps (44), these variations can adversely affect theefficacy of the blocking (FIG. 11 ). Although variation at the 5′ endhas similar effects on the 5′ blocking approach (FIG. 13 ), 5′ endvariants generally represent a smaller fraction of the total.Considering these limitations of the 3′ approach, it was decided tofocus on the 5′ approach for further studies.

Evaluating the Quantitative Performance of Blocked Libraries

To rigorously evaluate the effect of blocking hsa-miR-16-5p on themeasurement of the non-targeted miRNA species in the library, librarieswere generated from five human plasma samples. For each sample, twolibraries were generated that were unblocked, that is, subjected to theblocking ligation reaction without a blocking oligonucleotide.Additionally, two libraries were generated using a blockingoligonucleotide targeting hsa-miR-16-5p, for a total of four librariesper sample. The plasma samples were chosen to have a high degree ofhemolysis such that the blocking of hsa-miR-16-5p should have largeeffects.

For each sample, the replicate unblocked and hsa-miR-16-5p blockedlibraries were analyzed by using the application DESeq2 (see Materialsand Methods) to establish those miRNAs differentially affected by theblocking. The analysis was limited to miRNA species alone because theaccurate alignment of non-miRNA species was not universally preciseenough to allow for meaningful comparisons. As expected, members of themir-16 family are also blocked by this approach, due to sequencesimilarity at their 5′ ends (FIG. 4A-E). Interestingly, mir-16 familymembers has-miR-424-5p and hsa-miR-497-5p are not blocked, likelybecause they have a cytosine in the first position rather than theuracil that the other four members have (FIG. 4D). This is consistentwith the inability of T4 DNA Ligase to seal nicks at positions where abase pair mismatch is present at the nick site (44). It was observedthat in two of the five samples, a small number of non-targeted miRNAsare significantly lower in the blocked libraries as well (FIG. 4A-C).Some of these miRNAs have several bases of sequence similarity with theblocker oligonucleotide and would form a duplex with a one-base gapbetween its 5′ end and the 3′ end of the blocking oligonucleotide. Theinefficiency in sealing these gaps explains the moderate fold-changes.Other lowered species have no obvious sequence similarity. They are onlysignificantly lower in one sample (FIG. 4C) and have small fold changes,suggesting that possibly the FDR correction used by DESeq2 did notsufficiently correct the multiple hypothesis effect. Several miRNAs areactually significantly higher in the blocked libraries (FIGS. 4B and C).Presumably, these miRNAs were able to be more effectively ligated during5′ adaptor ligation in the absence of the highly abundant and preferredligation substrate hsa-miR-16-5p.

An important motivation for blocking hsa-miR-16-5p in these samples wasto increase detection of the low abundance species. With a basis of anequivalent number of aligned reads, comparison of the unblockedlibraries to the blocked libraries shows a marked increase in the numberof miRNA species detected at a variety of count thresholds in all fiveplasma samples (FIG. 5 ). At a commonly chosen cutoff of 10 counts,between 180 and 450 more miRNAs are detected at this threshold inblocked samples compared to unblocked samples. This improvement in thedetection of the low-abundance species was accomplished with negligibleincrease in library generation costs and no increase in sequencingcosts.

A critical concern is that the blocking protocol adversely impacts thereproducibility of the measurement of less abundant miRNAs and byextension, the ability to precisely measure differential expression.While it is reassuring that the measured abundances of the vast majorityof miRNAs are not affected by the blocking protocol (FIG. 4 ), it isimportant to note that given the known bias caused by the RNA ligasesused in the library generation protocol, the absolute abundance of themiRNAs in the library does not represent a strictly meaningfulmeasurement of the actual abundance in the sample. Nevertheless, thegoal of many studies is to measure differential expression betweensample groups. In these cases, the abundance of miRNAs needs only to bemeasured reproducibly.

To assess the reproducibility of the five human plasma sample libraries,the Spearman rho coefficient of correlation was calculated betweenreplicate libraries (FIG. 12 ). For all five samples, the Spearman rhowas higher for the blocked. However there was concern that correlationmay not be the best measure of reproducibility in these librariesbecause the biases introduced by the RNA ligases are consistent. Thus,correlation may be imposed upon a set of two libraries simply becausethey were subjected to the same bias. As an alternative, DESeq2 was usedto estimate the dispersions of each library based on its replicates (seeMaterials and Methods). DESeq2 proposes a negative binomial distributionas the appropriate distribution for count data (41) and estimates thedispersion as function of read depth. As seen in the data, thedispersion is generally highest at low counts and decreases withincreasing read depth (FIG. 6A). The blocked libraries show no greaterdispersion in any count regime, and may be less dispersed, particularlyin the middle-to-high count range.

Lastly, for blood-based biomarker studies, the ability to measuredifferential expression is paramount. Using DESeq2, the fold changeswere calculated between all possible pairs of samples (10 pairs)separately in both the unblocked libraries and the hsa-miR-16-5p blockedlibraries. The measurement of the fold change of miRNAs between twosamples is highly similar in the unblocked and hsa-miR-16-5p blockedlibraries (FIG. 21 ). With the exception of a few outliers, the vastmajority of fold changes scatter around the unity slope line (dashedline in FIG. 21 ). The Spearman coefficient of correlation wascalculated between the unblocked and hsa-miR-16-5p blocked libraries foreach of the 10 pairs. The coefficient values were very high, with a meanof 0.86 (FIG. 21 ). These data indicate that the blocking ofhsa-miR-16-5p has very little effect on the measurement of differentialexpression in this sample set.

Extension of the Blocking Technique to Other miRNAs and Multiplexing

To establish the ability of the blocking method to block species otherthan hsa-miR-16-5p, hsa-miR-451a was blocked with an appropriatelydesigned blocker oligonucleotide in a set of two human plasma samples. Aselection of hsa-miR-451a was made because it is also abundant in thelibraries prepared from human plasma samples and because it has beenimplicated to be derived from blood cells, like hsa-miR-16-5p (32). Theanalysis found hsa-miR-451a to be effectively blocked by this approachwith minimal off-target effects (FIGS. 7A and B). Other than theintended target, hsa-miR-451a, only hsa-miR-451b was significantlyaffected by the blocking. Although the canonical mature form ofhsa-miR-451b lacks significant sequence similarity to hsa-miR-451a, itwas found that the reads mapping to the hsa-miR-451b hairpin actuallyaligned near its stem-loop portion and do show sequence similarity withthe 5′ portion of the hsa-miR-451a canonical mature form.

The ability to combine blocking oligonucleotides in a single blockingreaction would allow for reduction of a chosen set of miRNAs. Blockingoligonucleotides were combined targeting hsa-miR-16-5p and hsa-miR-451ainto a single blocking reaction. The combination resulted in bothblocking oligonucleotides behaving as they did when in isolation (FIGS.7C and D). Other than the miRNAs expected to change based on the singleblocker experiments, only hsa-miR-503-5p was significantly downregulated, albeit with a small fold-change. hsa-miR-503-5p shares sevenbases of identical sequence with hsa-miR-16-5p on the 5′ end. Thecombination of blocking oligonucleotides had no discernible effect onthe total library yield. Overall, combining multiple blockingoligonucleotides seems to be a viable strategy. The extent to whichblocking oligonucleotides can be multiplexed is the subject of futureresearch.

DISCUSSION

Highly abundant and likely marginally informative miRNAs in NGS datasetsfrom human serum or plasma hinder one's ability to discover true smallRNA species functioning as biomarkers. This problem has been amelioratedby demonstrating a method to block miRNAs from representation insequencing libraries. This method uses inexpensive reagents and requiresno additional clean-up steps. Application of the method in human plasmasamples resulted in a robust blocking of hsa-miR-16-5p, an abundantblood cell contaminant.

As a result of this blocking, the read depth of low abundance miRNAs wasdramatically increased, leading to the detection of a greater number ofspecies and a more accurate measurement of differential expression.Off-target effects do occur based on sequence homology at the targetedend of the miRNA, in this case the 5′ end, especially within miRNAfamily members. However, these off-target effects are limited andpredictable. The method does not decrease the reproducibility of themeasurement of low abundance miRNAs and has no ill effects on themeasurement of differential expression.

The approach has been generalized by targeting a second miRNA,hsa-miR-451a. Again, the performance of the blocking method onhsa-miR-451a is specific and has very small effects on non-targetedspecies. Additionally, the combination of two blocking oligonucleotidestargeted to hsa-miR-16-5p and hsa-miR-451a in one blocking ligationreaction produced the same results seen by each one separately andwithout any interaction effects. This result implies the ability tocombine several blocking oligonucleotides into a single reaction,although it remains to be tested.

It is anticipated that this technology could fill a role in small RNAsequencing similar to that which ribosomal RNA and globin RNA reductionmethods have in messenger RNA sequencing. Although the research focusedon small RNA sequencing in human plasma samples, the method could beuseful in other tissue types as well. Custom pools of blockingoligonucleotides could be tailored to a particular application tomaximize the use of sequencing resources. Also, even though theexperimentation focused on the use of the Illumina platform, it isexpected that this method would be applicable to other platforms as longas the library generation method relies on the ligation of adaptorsdirectly to small RNAs. When it is anticipated that the small RNAs ofinterest will be rare and lowly expressed, as is likely true in manyapplications, the method offers a robust and cost-effective way toprecisely measure them.

Supplemental Methods

Overview

This protocol describes the preparation of multiplexed (barcoded)libraries of miRNA from total RNA samples suitable for sequencing on theIllumina HiSeq and GAII platforms. The total RNA must be prepared by atechnique that captures short RNA species (15nt-25nt). Acceptabletechniques are phenol-chloroform extraction followed by ethanolprecipitation or NorGen columns, amongst others. MicroRNA species in thesamples have an adaptor oligo (referred to as the 3′ adaptor) ligated totheir 3′ ends. Next a different oligo (referred to as the 5′ adaptor) isligated to the 5′ end. The 3′ adaptor provides a binding site for acomplementary RT primer. This allows for cDNA to be made from themiRNA-adaptor complex via reverse transcription. The cDNA is then usedas a template for several rounds of PCR. The PCR primers have long tails(˜30 nt) that extend the length of the product. The tails contain thebarcoding sequences (with an index read primer binding site), theIllumina sequencing primer site, and the Illumina cluster—generatingsequences.

Considerations

The first step, the 3′ ligation, is probably the most important step inthe protocol. It hinges on the use of a truncated form of T4 RNA ligase.The truncation renders the enzyme unable to use ATP for energy andinstead must use an already adenylated oligo as a substrate.Consequently, the 3′ adaptor oligo is adenylated on its 3′ end. If afully functional RNA ligase capable of using ATP were used in this step,it would ligate the various RNA species present into concatomers,instead of only ligating the adaptor to the target RNA species. However,one must appreciate that all RNA species are targets, not just themiRNAs, leading to the formation of numerous unintended ligationproducts. Not only must these products be removed prior to sequencing,the other RNA species distract the adaptor from the miRNA population.Although it is difficult to calculate directly, the effective efficiencyof ligation of the miRNAs present, in terms of the percent of miRNAsthat actually get ligated to a 3′ adaptor, is likely low. The 3′ adaptoralso has a 3-carbon spacer on its 5′ end. This is to prevent RNA frombeing ligating to its 5′ end in the subsequent 5′ ligation reaction,which uses full length T4 RNA ligase.

Although the unintended ligation products that occur when an RNAmolecule present in the sample other than a miRNA is ligated aresomewhat problematic, the most problematic product formed in thisprotocol arises when unligated 3′ adaptor is ligated to the 5′ adaptorcreating an adaptor dimer in the second ligation step.

This creates a short product that following PCR is highly complementaryto the intended miRNA ligation product. In fact, they only differ by theinternal ˜22 bp of the miRNA. The adaptor dimer will hybridizeefficiently to the intended miRNA product, making the separation of thetwo difficult. This problem is somewhat helped by hybridizing the RTprimer to the 3′ ligation product. Since the RT primer is complementaryto the entire length of the 3′ adaptor, the hybridization serves to bindsome of the unligated 3′ adaptor, preventing adaptor dimer formation inthe 5′ ligation reaction. While this technique reduces the formation ofthe adaptor dimer, much still persists and is present after PCR. It mustbe separated from the intended product by gel electrophoresis. However,because of the strong hybridization between the adaptor dimer and theintended miRNA product, the gel must be run under extremely denaturingconditions. To accomplish this, 10% acrylamide TBE-Urea gels are used.Furthermore, the gels are run in pre-heated buffer (90° C.). Although itis inconvenient to run the hot gels, the studies have shown that the 10%acrylamide TBE-Urea gels run at room temperature are not sufficientlydenaturing for this application.

Detailed Procedure

General Notes

For those steps in which multiple components are added to the reaction,best practice is to make a “master mix” of the components sufficient forall reactions being performed. The samples throughout the course of theprotocol should always be kept on ice or at ice temperature when notbeing otherwise incubated. In the development of this protocol, thesamples were kept in a metal block that was kept cool in a refrigeratorwhen not in use. (For simplicity, the protocol will say “on ice”,however.) Furthermore, the T4 RNA Ligase 2, truncated; the RNAseInhibitor, murine; the T4 RNA Ligase 1, the SuperScript II, and thePhusion PCR Master Mix should all be kept on ice. In the following step,“STOPPING POINT” is written at points where the protocol can be stoppedovernight. This protocol can be completed in 3 days. If theprecipitation overnight incubations at −30° C. are shortened to 2 hourincubations at −80° C., the protocol can be done in 2 busy days.

3′ Ligation

1. Prepare the following stock buffer, called “2×3” Ligation Buffer.This recipe is sufficient for many reactions and does not need to beprepared fresh each time the protocol is run. Store at −20° C. betweenuses:

250 μL 50% PEG 8000 (from T4 RNA Ligase 1 kit)

200 μL 10×T4 RNA Ligase Buffer (from T4 RNA Ligase 1 kit) 550 μL DNAse,RNAse free water

2. Make a stock solution of the spike-in controls. Make a large batchsuitable for multiple runs of this protocol. Store at −80° C. Theconcentrations listed here are suitable for human plasma samples.However, it is expected that the total input of the spike-ins will needto be adjusted for different sample types.Final Concentrations:20 μM miRNASeq Multiplex 22 bp Spike In 2 μM miRNASeq Multiplex 25 bpSpike In0.2 μM miRNASeq Multiplex 20 bp Spike In3. Combine the following in a 0.2 mL PCR tube.

-   -   1 μL 10 uM miRNASeq Multiplex 3′ Adaptor 1 μL Spike In stock        (from step 2)    -   4 μL of total RNA        4. Gently mix by flicking the tube and spin down the tube in a        tabletop mini-centrifuge. Incubate for 2 min at 70° C. in a        pre-heated thermal cycler. Immediately chill on ice following        incubation.        5. To each sample add the following: 10 μL 2×3′ Ligation Buffer        2 μL T4 RNA Ligase 2, truncated 1 μL RNAse Inhibitor, murine        6. Gently mix the components and spin down. Incubate for 1 hour        at 25° C. in a thermal cycler. (Note: Incubation times longer        than 1 hour have been shown to produce undesired products.)        Blocking Ligation        1. Pre-anneal the blocking oligonucleotide (do this every time).        Incubate a 0.5 μM blocking oligonucleotide stock in 1×T4 DNA        Ligase buffer as follows:        95° C. for 5 min, 65° C. for 5 min, 55° C. for 5 min, 45° C. for        5 min, 35° C. for 5 min, 25° C. for 5 min, 4° C. for infinity.        2. Make a master mix of the following:        1 μL of pre-annealed blocking oligonucleotide working stock 1 μL        of 10 mM ATP        1 μL of T4 DNA Ligase        3. Add 3 μL of the above master mix to each 3′ ligation        reaction.    -   4. Incubate at 30° C. for 1 hr followed by 65° C. for 10 min and        hold at 4° C.        RT Primer Hybridization, and 5′ Ligation        1. To the 3′ ligation product, add 1 μL of 10 uM miRNASeq        Multiplex RT Primer. Incubate as follows in a thermal cycler:        75° C. for 5 min, 37° C. for 30 min, 25° C. for 15 min, 4° C.        for inf        2. While the samples are incubating, thaw the 20 uM miRNASeq        Multiplex 5′ Adaptor. Once thawed, incubate the adaptor at        70° C. for 2 min and then immediately chill on ice.        3. A pool of 4 5′ adaptors is used in the next step. These are        an equimolar mix of miRNASeq Multiplex 5′ Adaptor Mod 1, 2, 3,        and 4 at 5 uM final concentration each, for a total adaptor        concentration of 20 uM.        4. When the samples are finished incubating, transfer them to        ice. Add the following:        0.64 μL T4 RNA Ligase 1        1 μL RNAse Inhibitor, murine        0.86 μL RNAse, DNAse free water        1 μL 20 uM miRNASeq Multiplex 5′ Adaptor Mod pool 1 μL 10×T4 RNA        Ligase Buffer (T4 RNA Ligase 1 kit) 1 μL 10 mM ATP (T4 RNA        Ligase 1 kit)        5. Mix gently and spin down briefly. Incubate the samples for 1        hour at 25° C. in a thermal cycler. STOPPING POINT (The samples        can be placed in −80° C. and left overnight after this step,        although it is ideal to take the samples through reverse        transcription before stopping)        Reverse Transcription and PCR        1. Setup the following reaction. The protocol up this point has        generated ˜26.5 μL of ligated product. Only 11 μL of the product        is carried forward, so that the remainder is available for a        repeat if needed. The unused product should be stored at −80° C.        4 μL 5×FS Buffer (SuperScript II kit) 2 μL 0.1 M DTT        (SuperScript II kit)        1 μL Deoxynucleotide Mix (10 mM each) 1 μL RNAse Inhibitor,        murine        1 μL SuperScript II (SuperScript II kit)        11 μL ligation product (from previous step)        2. Incubate the samples in a thermal cycler as follows: 42° C.        for 50 min        70° C. for 15 min 4° C. for inf        3. Add the following to each sample:        25 μL Phusion High-Fidelity PCR Master Mix        2.5 μL 20 uM miRNASeq Multiplex R Primer        To each individual sample add 2.5 μL of one of the twelve        different indexed miRNASeq Multiplex F Primers at 20 uM, being        sure to note which sample received which barcoded primer. Mix        the samples and spin down.        4. Incubate the samples in a thermal cycler as follows: 94° C.        for 30 s        15 cycles of: 94° C. for 10 s 72° C. for 45 s        65° C. for 5 min 4° C. for inf        STOPPING POINT (The samples can be stored at −20° C.)        Concentration, Gel Separation, and Purification        The gels run in this protocol are the Mini-PROTEAN format from        BioRad and run in the Mini-PROTEAN Tetra Cell gel system. It is        expected that using a different gel system would require that        extensive modifications be made to this protocol.        1. Transfer each sample to a 1.7 mL microcentrifuge tube. Add        250 μL of Buffer PB (MinElute Kit). Mix well and transfer to a        MinElute column placed in a 2 mL collection tube. Centrifuge for        1 min at max speed. Discard flow through.        2. Add 750 μL of Buffer PE (MinElute kit, ensure ethanol has        been added) to the MinElute column. Centrifuge for 1 min at max        speed. Discard flow through and place column back into the same        collection tube. Centrifuge again for 1 min at max speed.        3. Transfer the column to a clean 1.7 mL microcentrifuge tube.        Add 17.5 μL of RNAse, DNAse free water. Let stand for 5 min.        Centrifuge for 1 min at max speed. Discard column, keeping the        flow through in the microcentrifuge tube.        4. To each sample, add 17.5 μL of 2×TBE—Urea Sample Buffer. Mix        well and spin down. Set the samples aside at room temperature.        5. Prepare DNA ladder working solutions. This recipe makes        enough for several runs and need not be made fresh. Store at 4°        C.        20 bp Ladder    -   200 μL 2×TBE-Urea Sample Buffer 180 μL DNAse, RNAse free water    -   20 μL 20 bp DNA Ladder stock solution (Bayou BioLabs)    -   100 bp Ladder    -   200 μL 2×TBE—Urea Sample Buffer 190 μL DNAse, RNAse free water    -   10 μL 100 bp DNA Ladder stock solution (NEB)        6. At this point in the protocol, a hot gel will be run. Since        this involves using nea-boiling TBE buffer, extreme caution        should be used. Additionally, protective equipment such as        aprons and gloves should be worn.        7. Preheat a heating block to 95° C.        8. Make 1×TBE buffer from 10×TBE buffer stock. Make 1 liter,        sufficient for one or two gels. A single gel, can accommodate        four samples with no spacer lane between samples. Each sample        will be split and run in two lanes to avoid interference from        the adaptor dimer. It is not recommended to run more than two        gels at a time.        9. Pre-warm 10% TBE-Urea Mini-PROTEAN gel(s) in hot tap water        (no hotter than what comes out of the tap). Leave them in their        packaging and weigh them down so they don't float. Also, warm        the gel holder in the water.        10. In a microwave, heat 900 mL of 1×TBE buffer split into        aliquots of 450 mL in two 500 mL Pyrex beakers with Saran wrap        partially covering the top to 80-85° C. Heat in increments of        2-5 min (depending on microwave power). Between heating        increments, carefully stir the buffer with a thermometer and        check the temperature. Do not boil the buffer.        11. When the heating of the buffer is nearing completion, place        the samples into the preheated heating block at 95° C. Also        place the 20 bp and 100 bp working solutions in the heating        block. Ensure that every sample resides at 95° C. for at least        two minutes before it is loaded onto the gel. It is not        detrimental for the samples to remain in the heating block for        more than 2 min, up to −30 min.        12. Remove the gels and gel holder from the warm water. Remove        the gels from their packaging, ensuring to remove the green tape        at the bottom of the gel and the lane comb. Assemble the gels in        the gel holder.        13. Pour the now hot 1×TBE buffer (80-85° C.) into the gel        assembly, filling it to the top.        14. With a p20 set to 15 μL, pipet up and down in each well of        the gel. This is to remove any urea that often crystalizes in        the wells during storage. Remove any bubbles in the wells.        15. Remove the two ladder tubes (carefully, they are hot). Spin        them down briefly in a tabletop mini centrifuge. Add 15 μL of        the 20 bp ladder to lane 1 of the gel, pipetting carefully to        avoid contamination of other lanes. The tube may make a “pop”        when opened. Add 15 μL of the 100 bp ladder to lane 2.        16. Remove a pair of sample tubes from the heating block. Spin        them down briefly in a tabletop mini centrifuge. Load two 15 μL        aliquots of each sample into two adjacent lanes of the gel.        Repeat for all of the samples. Work quickly because the gel is        cooling, but carefully and deliberately.        17. Once all the samples are loaded, gently place the gel        assembly into the gel box. Re-heat the remaining 1×TBE buffer to        90° C. in the microwave. Pour all the remaining 1×TBE into the        gel box (not inside the gel assembly).        18. With a 10 mL pipet, top-off the buffer inside of the gel        assembly with buffer in the gel box, filling it as near to the        top as possible. This is important because the hot buffer will        evaporate during the course of the run.        19. Begin running the gel at 200V. Closely monitor the current.        If the current begins to rise more than 10 mA from the initial        current (this is likely to happen), turn the voltage down 10V to        190V. Continue to monitor the current and adjust the voltage        lower until the current stabilizes. However, do not run the gel        below 160V. The current rises because the gel and buffer are        hot. The conductivity of the system is much higher than when run        at room temperature. The increased conductivity allows more        current to flow, which in turn heats the gel, further increasing        conductivity, and creating a positive feedback loop. Thus, the        current must be monitored closely during the run. Under these        conditions, the gel should be run for 45 minutes.        20. Turn off the power source and disassemble the gel box. Allow        the gels to cool on the bench top prior to opening their plastic        cases. While the gels are cooling, for each gel, add 50 mL of        1×TBE to a suitably sized gel staining container. Add 5 μL of        SYBR Gold 10,000× stock to each 50 mL TBE aliquot and mix. Wrap        the container in aluminum foil to protect it from light. Open        the plastic case of the now cooled gel and place the gel into        the staining container with the TBE and SYBR Gold. Re-cover the        container with the aluminum foil and rock on a gel rocker for 10        minutes.        21. While the gel is staining, prepare the following for each        sample. With a 20-gauge needle, poke a hole in the bottom of a        0.5 mL microcentrifuge tube. Place this tube into a 1.7 mL        centrifuge tube.        22. Place a sheet of Saran wrap on a UV-transilluminator.        Transfer the gel from the staining solution onto the Saran wrap        sheet. Capture an image of the gel under UV illumination with an        appropriate gel visualization system (i.e. UVP EC3 Imaging        System).        23. Transfer the gel by picking up the Saran wrap to a        UV-transilluminator that can be accessed for subsequent gel        excision steps (maybe the same as where the image was taken).        With razor blades and forceps, carefully excise the 135 bp band        for each sample. Since each sample was loaded in two aliquots in        adjacent lanes, cut both bands from the same sample out        together. Replace the razor blades and forceps after every time        they touch the gel to avoid cross-contamination. Place the gel        fragments into the 0.5 mL microcentrifuge tube with the hole in        the bottom.        24. Transfer the 0.5 mL microcentrifuge tubes nested in 1.7 mL        microcentrifuge tubes containing the gel pieces into a        microcentrifuge. Spin at max speed for 1 min. The gel fragment        should be in the bottom of 1.7 mL microcentrifuge tube in small        pieces. If some of the gel fragment is retained in the 0.5 mL,        spin at max speed for another minute.        25. Prepare the following stock, called “Soaking Solution”. This        recipe makes enough for many samples as does not need to be        prepared fresh every time. Store at room temperature.    -   2 mL 5M Ammonium Acetate 2 mL 1% SDS solution    -   4 μL 0.5M EDTA    -   16 mL RNAse, DNAse free water        26. Add 300 μL of the Soaking Solution to each sample. Incubate        with agitation at 70° C. for 2 hours.        21. Transfer each sample (including gel pieces) to a Spin-X        Centrifuge Tube Filter, 0.22 urn Cellulose Acetate, sitting in        its accompanying microcentrifuge tube. Spin in a microcentrifuge        at max speed for 1 min.        28. Transfer the flow-though to a new 1.7 mL microcentrifuge        tube. Add 1 μL of 10 ug/μL glycogen. Add 300 μL of 100%        isopropanol. Vortex and spin down briefly. Incubate overnight at        −30° C. STOPPING POINT (The samples can be kept in the        precipitating conditions at −3° C. for several days.)        29. Spin the samples in a refrigerated centrifuge (4° C.) for 20        min at 14,000 rpm (max speed). Again, place the hinges of the        tubes outward so that the location of the pellet in predictable.        30. While the samples are in the centrifuge, chill an aliquot of        80% ethanol by place in it in ice water or by some other        suitable method.        31. After centrifugation, pipet off the supernatant. Using a        p200, place the tip of the pipet near the bottom of the tube        away from the hinge side and gently remove the liquid. Add 100        μL of the chilled 80% ethanol and centrifuge again in a        refrigerated centrifuge (4 C) for 10 min at 14,000 rpm (max        speed).        32. Again carefully remove the supernatant with the p200 as        described above. After removing as much as possible with the        p200, use a p20 to get the remainder, leaving behind as little        liquid as possible.        33. Resuspend the pellet in 10 μL of EB buffer (MinElute Kit).        Measure the concentration of the sample with a suitable method        (QBit HS DNA is preferred with 1 μL of sample input). The sample        is ready for sequencing. Typically, this protocol yields 10 μL        of 1-4 ng/μL product, depending on sample input mass and sample        type. Although it depends on the level of multiplexing, 0.5        ng/μL or higher libraries are concentrated enough for        sequencing. If a lower yield is expected, the pellet can be        resuspended in a lower volume to yield a higher concentration        product.        It is highly recommended to run KAPA qPCR to quantify library        concentrations before sequencing.

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2. Working Example #2: Determination of Relative Counts of UnwantedmiRNA in Blocked and Unblocked Samples

Initial attempts to use blocking nucleic acids that ligate to the 3′ endof the unwanted miRNA were unsuccessful, resulting in miRNA librarieswith low efficiency of adaptor binding. Eventually it was hypothesizedthat residual ATP used in the T4 DNA ligation reaction was inhibitingthe effectiveness of T4 RNA ligase 2, truncated. The following protocolwas developed to solve the problem.

3′ Blocking Protocol

Initially, 1 μl 0.5 μM miRNA blocking oligonucleotide was incubated forfive minutes at each of the following temperatures: 95° C., 65° C., 55°C., 45° C., and 35° C. to ensure the proper formation of the hairpinstructure. The incubated blocking oligonucleotide was combined with 4 μlof isolated RNA containing microRNAs along with 1 μl T4 DNA Ligase(NEB), 1 μl 10×T4 DNA Ligase Buffer (NEB), 1 μl Murine RNAse inhibitor(NEB), and 2 μl of water and incubated for one hour at 30° C. and 15minutes at 65° C. to anneal and block the unwanted miRNA from furtherreactions. After incubation, the reaction products were isolated using acolumn capable of binding microRNAs and eluted in 20 μl of water inorder to remove the ATP present from the 10×T4 DNA Ligase buffer, whichis inhibitory to T4 RNA Ligase 2, truncated. After the column clean-up,10 μl of the column eluate, 1 μl of 10 μM 3′ adaptor, 1 μl T4 RNA Ligase2, truncated (NEB), 1 μl Murine RNAse inhibitor (NEB), 2.5 μl 50% PEG8000 (NEB), 2 μl 10×T4 RNA Ligase Buffer (RNA), and 0.5 μl of water werecombined to a final volume of 19 μl and incubated at 25° C. for onehour. One μl of 10 μM reverse transcription primer was annealed to the3′ adaptor ligation product for five minutes at 75° C., 30 minutes at37° C., and 15 minutes at 25° C. prior to the addition of the 5′ adaptorin order to reduce formation of adaptor-dimer products. One μl of 20 μMpooled 5′ adaptor was incubated for two minutes at 70° C. and thencombined with the previous reaction as well as 1 μl T4 RNA Ligase 1(NEB), 1 μl Murine RNAse Inhibitor (NEB) 1 μl 10 mM ATP, and 1 μl 10×T4RNA Ligase Buffer, and incubated for one hour at 25° C. Ligated reactionproducts were reverse transcribed by combining 11 μl of the ligationreaction with 1 μl SuperScript II (Invitrogen), 4 μl 5× First StrandReaction Buffer (Invitrogen), 2 μl of 100 mM DTT (Invitrogen), 1 μlMurine RNAse Inhibitor (NEB), and 1 μl 10 mM dNTPs each and incubatedfor 50 minutes at 42° C. and 15 minutes at 70° C. The resulting cDNAcontaining reaction was amplified via PCR by adding 25 μl of 2× PhusionHigh-Fidelity PCR Master Mix (NEB) along 2.5 μl each of the 20 μMforward and reverse primers. The thermal cycling conditions were 94° C.for 30 sec, followed by 15 cycles of 94° C. for 10 sec, and 72° C. for45 sec, and a final extension at 65° C. for five minutes. Libraries werecleaned and concentrated using a MinElute PCR Purification Kit (Qiagen),following the manufacturer's instructions, and eluted into a finalvolume of 20 μl. Libraries were separated on a TBE-Urea 10 percentacrylamide gel (Bio-Rad) with warm buffer for 50 minutes. The bandcorresponding to miRNAs (˜135-145 base pairs) was excised, eluted fromthe gel, precipitated, and resuspended in 10 μl of EB Buffer (Qiagen).Small RNA library concentration was quantified by the LibraryQuantification Kit—Illumina/ABI Prism (KAPA Biosystems) and sequenced ona HiSeq2000 or a MiSeq according to standard Illumina protocols.

Study of Blocking Efficiency Using Singular and Pooled 5′ and 3′Blockers

Using the same methods described above, miRNA libraries of human heartmiRNA were constructed using the blocking protocol and without theblocking protocol. The improved 3′ blocking protocol from the previoussection was used to test the efficacy of a 3′ blocker to miR-16.

Comparisons were made to libraries without blocker and libraries madeusing an hsa-miR-16-5p 3′ blocker (SEQ ID NO: 13 with a 3′ C3phosphoramidite spacer), an hsa-miR-26a-5p 5′ blocker (SEQ ID NO: 2 witha 5′ C3 phosphoramidite spacer), and an hsa-miR-486-5p 5′ blocker (SEQID NO: 1 with a 5′ C3 phosphoramidite spacer). As shown in FIG. 14-16respectively, in each case the normalized counts of the blocked miRNAwas reduced significantly compared to the control.

The blocking protocol was then tested using a mixed pool of blockers. Apool of blockers was prepared containing an hsa-miR-16-5p 3′ blocker(SEQ ID NO: 13 with a 3′ C3 phosphoramidite spacer), an hsa-miR-26a-5p5′ blocker (SEQ ID NO: 2 with a 5′ C3 phosphoramidite spacer),hsa-miR-451-5p 5′ blocker (SEQ ID NO: 3 with a 5′ C3 phosphoramiditespacer) and an hsa-miR-486-5p 5′ blocker (SEQ ID NO: 1 with a 5′ C3phosphoramidite spacer). miRNA libraries of human heart miRNA wereconstructed using the blocking protocol with the pool and without theblocking protocol. The abundances of each of hsa-miR-16-5p,hsa-miR-26a-5p, and hsa-miR-486-5p were measured. As can be seen in FIG.17-20 , the pooled blockers significantly reduced the abundance of eachof the unwanted miRNAs in the resultant library.

G. Supported Embodiments

This disclosure specifically but non-exclusively supports claims to thefollowing embodiments: Emb 1. A blocking nucleic acid for use inreducing the abundance of an unwanted micro-RNA (miRNA) in an miRNAlibrary, the blocking nucleic acid comprising: (a) a Crick strand havinga 3′ end and a 5′ end; (b) a single stranded complementary region at oneof the 5′ end of the Crick strand or the 3′ end of the Crick strand,that anneals with a binding region at a first end of the unwanted miRNAunder stringent conditions, wherein said first end is the 5′ end or the3′ end of the unwanted miRNA; (c) a double-stranded region on the Crickstrand adjacent to the complementary region, the double-stranded regioncomprising a Watson strand that is annealed to the Crick strand, theWatson strand having a 5′ end and a 3′ end; (d) a first blocking moietylinked to the 3′ end of the Crick strand, wherein the first blockingmoiety cannot serve as a substrate for ligases; (e) a second blockingmoiety linked to the 5′ end of the Crick strand, wherein the secondblocking moiety cannot serve as a substrate for ligases; (f) a thirdblocking moiety linked to the 3′ end of the Watson strand if thecomplementary region is at the 3′ end of the Crick strand, or linked tothe 5′ end of the Watson strand if the complementary region is at the 5′end of the Crick strand, wherein the third blocking moiety cannot serveas a substrate for ligases; and (g) a ligative terminal end on theWatson strand, the ligative terminal end located at the 3′ end of theWatson strand if the complementary region is at the 5′ end of the Crickstrand, or at the 5′ end of the Watson strand if the complementaryregion is at the 3′ end of the Crick strand. Emb 2. A blocking nucleicacid for use in reducing the abundance of an unwanted micro-RNA (miRNA)in an miRNA library, the blocking nucleic acid comprising: (a) a 5′ endof the blocking nucleic acid and a 3′ end of the blocking nucleic acid;(b) a single-stranded complementary region at one of the 5′ end of theblocking nucleic acid or the 3′ end of the blocking nucleic acid, thatanneals with a binding region at a first end of the unwanted miRNA understringent conditions, wherein said first end is either the 5′ end or the3′ end of the unwanted miRNA, and wherein the complementary region has aterminal end; (c) a hairpin loop forming region adjacent to thecomplimentary region, the hairpin loop forming region having a ligativeterminal end; and (d) a first blocking moiety linked to the terminal endof the complementary region, in which said first blocking moiety cannotserve as a substrate for ligases. Emb 3. The blocking nucleic acid ofany one of the above, wherein the complementary region anneals with thebinding region under highly stringent conditions. Emb 4. The blockingnucleic acid of any one of the above, wherein the complementary regionanneals with the binding region under maximally stringent conditions Emb5. The blocking nucleic acid of any one of the above, wherein theblocking nucleic acid comprises a linker group between the firstblocking moiety and the complementary region. Emb 6. The blockingnucleic acid of any one of the above, wherein the blocking nucleic acidcomprises a linker group between the first blocking moiety and thecomplementary region, wherein the linker group is selected from thegroup consisting of: Spacer 9 (triethylene glycol) and Spacer 18(hexa-ethyleneglycol). Emb 7. The blocking nucleic acid of any one ofthe above, wherein the complementary region is 5-50 nucleotides inlength. Emb 8. The blocking nucleic acid of any one of the above,wherein the complementary region is 8-20 nucleotides in length. Emb 9.The blocking nucleic acid of any one of the above, wherein thecomplementary region is 10-15 nucleotides in length. Emb 10. Theblocking nucleic acid of any one of the above, wherein the complementaryregion comprises a sequence having at least 90% identity with positions1-12 of one of SEQ ID NO: 1-4. Emb 11. The blocking nucleic acid of anyone of the above, wherein the complementary region comprises a sequencehaving greater than 95% identity with positions 1-12 of one of SEQ IDNO: 1-4. Emb 12. The blocking nucleic acid of any one of the above,wherein the complementary region comprises a sequence having at least90% identity with positions 1-12 of SEQ ID NO: 4. Emb 13. The blockingnucleic acid of any one of the above, wherein the complementary regioncomprises a sequence having greater than 95% identity with positions1-12 of SEQ ID NO: 4. Emb 14. The blocking nucleic acid of any one ofthe above, wherein the first blocking moiety is a modified nucleotidethat either lacks an available 5′ phosphate group, lacks an available 3′hydroxyl group, or both. Emb 15. The blocking nucleic acid of any one ofthe above, wherein the first blocking moiety is selected from the groupconsisting of: an inverted deoxynucleotide, dideoxynucleotide, aninverted dideoxynucleotide, C3 spacer (phosphoramidite), Spacer 9(triethylene glycol), propyl group, propanol group, and Spacer 18(hexa-ethyleneglycol). Emb 16. The blocking nucleic acid of Emb 2,wherein said hairpin loop forming region group comprises a sequencehaving at least 90% identity with SEQ ID NO: 5. Emb 17. The blockingnucleic acid of any one of Emb 2 or 16, wherein said hairpin loopforming region group comprises a sequence having greater than 95%identity with SEQ ID NO: 5. Emb 18. The blocking nucleic acid of any oneof the above, wherein the complementary region anneals under stringentconditions with at least 5 consecutive bases of at least one of SEQ IDNOS: 6-11. Emb 19. The blocking nucleic acid of any one of the above,wherein the complementary region anneals under stringent conditions withat least 8 consecutive bases of at least one of SEQ ID NOS: 6-11. Emb20. The blocking nucleic acid of any one of the above, wherein thecomplementary region anneals under stringent conditions with at least 10consecutive bases of at least one of SEQ ID NOS: 6-11. Emb 21. Theblocking nucleic acid of any one of the above, wherein the complementaryregion anneals under stringent conditions with positions 1-8 of at leastone of SEQ ID NO:6 and SEQ ID NO: 10. Emb 22. The blocking nucleic acidof any one of the above, wherein the complementary region anneals understringent conditions with positions 1-9 of at least one of SEQ ID NOS:6-11. Emb 23. The blocking nucleic acid of any one of the above, whereinthe complementary region anneals under highly stringent conditions withat least 5 consecutive bases of at least one of SEQ ID NOS: 6-11. Emb24. The blocking nucleic acid of any one of the above, wherein thecomplementary region anneals under highly stringent conditions with atleast 8 consecutive bases of at least one of SEQ ID NOS: 6-11. Emb 25.The blocking nucleic acid of any one of the above, wherein thecomplementary region anneals under highly stringent conditions with atleast 10 consecutive bases of at least one of SEQ ID NOS: 6-11. Emb 26.The blocking nucleic acid of any one of the above, wherein thecomplementary region anneals under highly stringent conditions withpositions 1-8 of at least one of SEQ ID NO:6 and SEQ ID NO: 10. Emb 27.The blocking nucleic acid of any one of the above, wherein thecomplementary region anneals under highly stringent conditions withpositions 1-9 of at least one of SEQ ID NOS: 6-11. Emb 28. The blockingnucleic acid of any one of the above, wherein the complementary regionanneals under maximally stringent conditions with at least 5 consecutivebases of at least one of SEQ ID NOS: 6-11. Emb 29. The blocking nucleicacid of any one of the above, wherein the complementary region annealsunder maximally stringent conditions with at least 8 consecutive basesof at least one of SEQ ID NOS: 6-11. Emb 30. The blocking nucleic acidof any one of the above, wherein the complementary region anneals undermaximally stringent conditions with at least 10 consecutive bases of atleast one of SEQ ID NOS: 6-11. Emb 31. The blocking nucleic acid of anyone of the above, wherein the complementary region anneals undermaximally stringent conditions with positions 1-8 of at least one of SEQID NO:6 and SEQ ID NO: 10. Emb 32. The blocking nucleic acid of any oneof the above, wherein the complementary region anneals under maximallystringent conditions with positions 1-9 of at least one of SEQ ID NOS:6-11. Emb 33. The blocking nucleic acid of any one of the above, whereinthe blocking nucleic acid is composed of a nucleic acid selected fromthe group consisting of: DNA, RNA, locked nucleic acid, and bridgednucleic acid. Emb 34. The blocking nucleic acid of any one of Emb 2, 16and 17 wherein the blocking nucleic acid is a DNA molecule comprising asequence having at least 90% identity with at least one of: SEQ ID NOS:1-4. Emb 35. The blocking nucleic acid of any one of Emb 2, 16, 17, and34 wherein the blocking nucleic acid is a DNA molecule comprising asequence having greater than 95% identity with at least one of: SEQ IDNOS: 1-4. Emb 36. The blocking nucleic acid of any one of Emb 2, 16, 17,34, and 35 wherein the blocking nucleic acid is a DNA moleculecomprising a sequence having at least 90% identity with SEQ ID NO: 4.Emb 37. The blocking nucleic acid of any one of Emb 2, 16, 17, and34-36, wherein the blocking nucleic acid is a DNA molecule comprising asequence having greater than 95% identity with SEQ ID NO: 4. Emb 38. Theblocking nucleic acid of any one of Emb 2, 16 and 17, wherein theblocking nucleic acid is a DNA molecule comprising a sequence having atleast 90% identity with SEQ ID NO: 13. Emb 39. The blocking nucleic acidof any one of Emb 2, 16, 17, and 38 wherein the blocking nucleic acid isa DNA molecule comprising a sequence having greater than 95% identitywith SEQ ID NO: 13. Emb 40. The blocking nucleic acid of any one of theabove, wherein the ligative terminal end is a nucleotide having one ofan available 5′ phosphate group or an available 3′ hydroxyl group. Emb41. A method of preventing a unwanted micro-RNA (miRNA) fromparticipating in reverse transcription polymerase chain reactions(RT-PCR), the unwanted miRNA having a 5′ end and a 3′ end, the methodcomprising: annealing the complementary region of the blocking nucleicacid of any one Emb 1-Emb 40 to the binding site at the first end of theunwanted miRNA, wherein the first end of the unwanted miRNA is one ofthe 5′ end or the 3′ end. Emb 42. The method of Emb 41, whereinannealing is conducted under stringent conditions. Emb 43. The method ofany one of Emb 41-42, wherein annealing is conducted under highlystringent conditions. Emb 44. The method of any one of Emb 41-43,wherein annealing is conducted under maximally stringent conditions. Emb45. The method of any one of Emb 41-44, comprising ligating the blockingnucleic acid to the first end of the unwanted miRNA. Emb 46. The methodof any one of Emb 41-45, wherein the first end of the unwanted miRNA isthe 5′ end and the complementary region is at the 5′ end of the blockingnucleic acid. Emb 47. The method of any one of Emb 41-46, wherein thefirst end of the unwanted miRNA is the 3′ end and the complementaryregion is at the 3′ end of the blocking nucleic acid. Emb 48. The methodof any one of Emb 41-47, comprising ligating the blocking nucleic acidto the first end of the unwanted miRNA, wherein the ligating step isperformed using a DNA/RNA ligase. Emb 49. The method of any one of Emb41-48, comprising ligating the blocking nucleic acid to the first end ofthe unwanted miRNA, wherein the ligating step is performed using a T4DNA ligase. Emb 50. The method of any one of Emb 41-49, wherein theunwanted miRNA is selected from the group consisting of: mir-16,mir-15a, mir-15b, mir-195, mir-424, mir-497, mir-486, mir-451, andmir-26. Emb 51. A blocked micro RNA (miRNA) complex that is the productof the method of any one of Emb 41-50. Emb 52. A method of reducing theabundance of a unwanted micro-RNA (miRNA) in an miRNA library, theunwanted miRNA having a 5′ end and a 3′ end, the method comprising: (a)purifying RNA from a sample comprising a plurality of miRNAs; (b)introducing an adenylated nucleic acid adapter and a first DNA/RNAligase under conditions to allow the adenylated nucleic acid adapter toligate to the 3′ ends of the plurality of miRNAs; (c) introducing theblocking nucleic acid of any one Emb 1-Emb 40 under conditions to allowthe complementary region of the blocking nucleic acid to anneal to thebinding region of the unwanted miRNA, to produce a blocked sample; (d)introducing an RNA adapter and an RNA ligase under conditions to allowthe RNA adapter to ligate the 5′ end of the plurality of miRNAs; (e)introducing a reverse transcriptase to the blocked sample underconditions to allow reverse transcription of the plurality of miRNAs, toproduce a cDNA sample; and (f) performing the polymerase chain reaction(PCR) on the cDNA sample to produce the miRNA library with reducedabundance of unwanted miRNA. Emb 53. The method of Emb 52, comprisingintroducing a second DNA/RNA ligase under conditions to allow theblocking nucleic acid to ligate to one of the 5′ end and the 3′ end ofthe unwanted miRNA. Emb 54. The method of Emb 53, wherein the secondDNA/RNA ligase is T4 DNA ligase. Emb 55. The method of any one of Emb52-54, comprising incubating the blocking nucleic acid with the unwantedmiRNA under stringent conditions to allow the complementary region ofthe blocking nucleic acid to anneal to the binding region of theunwanted miRNA. Emb 56. The method of any one of Emb 52-55, comprisingincubating the blocking nucleic acid with the unwanted miRNA underhighly stringent conditions to allow the complementary region of theblocking nucleic acid to anneal to the binding region of the unwantedmiRNA. Emb 57. The method of any one of Emb 52-56, comprising incubatingthe blocking nucleic acid with the unwanted miRNA under maximallystringent conditions to allow the complementary region of the blockingnucleic acid to anneal to the binding region of the unwanted miRNA. Emb58. The method of any one of Emb 52-57, wherein said adenylated nucleicacid adapter comprises a reverse transcriptase primer binding site. Emb59. The method of any one of Emb 52-58, wherein the adenylated nucleicacid adapter is an adenylated DNA adapter. Emb 60. The method of any oneof Emb 52-59, wherein the adenylated nucleic acid adapter is anadenylated RNA adapter. Emb 61. The method of any one of Emb 52-60,wherein the first DNA/RNA ligase is T4 ligase 2, truncated. Emb 62. Themethod of any one of Emb 52-61, wherein the RNA adapter is 5-30 basepairs in length. Emb 63. The method of any one of Emb 52-62, wherein theRNA adapter is 18-22 base pairs in length. Emb 64. The method of any oneof Emb 52-63, wherein the RNA ligase is T4 RNA ligase 1. Emb 65. Themethod of any one of Emb 52-64, wherein step (c) is performed before atleast one of steps (b) and (d). Emb 66. The method of any one of Emb52-65, wherein the binding region of the unwanted miRNA is the 5′ end ofthe unwanted miRNA. Emb 67. The method of any one of Emb 52-66, whereinthe binding region of the unwanted miRNA is the 5′ end of the unwantedmiRNA, and wherein the steps are performed in the following order: (a),(b), (c), (d), (e), and (f). Emb 68. The method of any one of Emb 52-67,wherein the binding region of the unwanted miRNA is the 3′ end of theunwanted miRNA. Emb 69. The method of any one of Emb 52-68, wherein thebinding region of the unwanted miRNA is the 3′ end of the unwantedmiRNA; wherein step (c) is performed before step (b); and wherein theconcentration of ATP is reduced between steps (c) and (b). Emb 70. AnmiRNA library with reduced abundance of an unwanted miRNA that is theproduct of the method of any one of Emb 52-69. Emb 71. The miRNA libraryof Emb 70, wherein the abundance of unwanted miRNA has been reduced atleast by an amount selected from the group consisting of: 50%, 60%, 70%,75%, 80%, 85%, 90%, 95%, 97.5%, 98%, 99%, 99.5%, 99%, and 100%. Emb 72.A kit for reducing the frequency of an miRNA in an miRNA library, thekit comprising any the blocking nucleic acids of any one of Emb 1-40.Emb 73. The kit of Emb 72, comprising a container of a DNA/RNA ligasecapable of ligating DNA to RNA when annealed. Emb 74. The kit of any oneof Emb 72-73, comprising a container of T4 DNA ligase. Emb 75. The kitof any one of The kit of any one of Emb 72-74, comprising a container ofan RNA/RNA ligase. Emb 76. The kit of any one of The kit of any one ofEmb 72-75, comprising a container of T4 RNA ligase 1. Emb 77. The kit ofany one of The kit of any one of Emb 72-75, comprising a container of anRNA/DNA ligase. Emb 78. The kit of any one of Emb The kit of any one ofEmb 72-77, comprising a container of T4 RNA ligase 2 truncated. Emb 79.The kit of any one of Emb The kit of any one of Emb 72-78, comprising acontainer of a reverse transcriptase. Emb 80. The kit of any one of EmbThe kit of any one of Emb 72-79, comprising a container of adenylatednucleic acid adapter. Emb 81. The kit of any one of Emb The kit of anyone of Emb 72-80, comprising a container of adenylated nucleic acidadapter, wherein said adenylated nucleic acid adapter comprises areverse transcriptase primer binding site. Emb 82. The kit of any one ofEmb The kit of any one of Emb 72-81, comprising a container ofadenylated nucleic acid adapter, wherein said adenylated nucleic acidadapter is an adenylated DNA adapter. Emb 83. The kit of any one of EmbThe kit of any one of Emb 72-82, comprising a container of adenylatednucleic acid adapter, wherein said adenylated nucleic acid adapter is anadenylated RNA adapter. Emb 84. The kit of any one of Emb The kit of anyone of Emb 72-83, comprising a container of an RNA adapter. Emb 85. Thekit of any one of Emb The kit of any one of Emb 72-84, comprising aplurality of DNA primers, a nucleotide solution, a PCR buffer, and athermophilic DNA polymerase. Emb 86. A blocked micro RNA (miRNA)complex, comprising an miRNA annealed to the blocking nucleic acid ofany one of Emb 1-40 at the binding region of the miRNA, wherein thefirst end is one of the 5′ end or the 3′ end. Emb 87. The blocked miRNAcomplex of Emb 86, wherein the first end is the 5′ end of the miRNA. Emb88. The blocked miRNA complex of Emb 86, wherein the first end is the 3′end of the miRNA. Emb 89. The blocked miRNA complex of any one of Emb86-88, wherein the miRNA is selected from the group consisting of:mir-16, mir-15a, mir-15b, mir-195, mir-424, mir-497, mir-486, mir-451,and mir-26. Emb 90. A nucleic acid molecule comprising a sequence havingat least 90% identity with one of SEQ ID NOS: 1-4 AND 13. Emb 91. Anucleic acid molecule comprising a sequence having greater than 95%identity with one of SEQ ID NOS: 1-4 AND 13. Emb 92. A nucleic acidmolecule comprising a sequence that is one of SEQ ID NOS: 1-4 AND 13.Emb 93. A nucleic acid molecule that anneals under stringent conditionswith the nucleic acid molecule of any one of Emb 90-92. Emb 94. Anucleic acid molecule that anneals under highly stringent conditionswith the nucleic acid molecule of any one of 90-92. Emb 95. A nucleicacid molecule that anneals under maximally stringent conditions with thenucleic acid molecule of any one of Emb 90-92. Emb 96. A cell comprisingany one of the nucleic acid molecules of any one of Emb 90-95. Emb 97.The cell of Emb 96, wherein the cell is a prokaryotic cell. Emb 97. Avector comprising any one of the nucleic acid molecules of any one ofEmb 90-95.

H. Conclusions

It is to be understood that any given elements of the disclosedembodiments of the invention may be embodied in a single structure, asingle step, a single substance, or the like. Similarly, a given elementof the disclosed embodiment may be embodied in multiple structures,steps, substances, or the like.

The foregoing description illustrates and describes the processes,machines, manufactures, compositions of matter, and other teachings ofthe present disclosure. Additionally, the disclosure shows and describesonly certain embodiments of the processes, machines, manufactures,compositions of matter, and other teachings disclosed, but, as mentionedabove, it is to be understood that the teachings of the presentdisclosure are capable of use in various other combinations,modifications, and environments and are capable of changes ormodifications within the scope of the teachings as expressed herein,commensurate with the skill and/or knowledge of a person having ordinaryskill in the relevant art. The embodiments described hereinabove arefurther intended to explain certain best modes known of practicing theprocesses, machines, manufactures, compositions of matter, and otherteachings of the present disclosure and to enable others skilled in theart to utilize the teachings of the present disclosure in such, orother, embodiments and with the various modifications required by theparticular applications or uses. Accordingly, the processes, machines,manufactures, compositions of matter, and other teachings of the presentdisclosure are not intended to limit the exact embodiments and examplesdisclosed herein. Any section headings herein are provided only forconsistency with the suggestions of 37 C.F.R. § 1.77 or otherwise toprovide organizational queues. These headings shall not limit orcharacterize the invention(s) set forth herein.

TABLE 2 SEQUENCE LISTING KEY SEQ ID NO DESCRIPTION 1 Example of DNAportion of 5′ blocking molecule that targets mir-486 2 Example of DNAportion of 5′ blocking molecule that targets mir-26 3 Example of DNAportion of 5′ blocking molecule that targets mir-451 4 Example of DNAportion of 5′ blocking molecule that targets mir-16 5 Consensus sequencebetween SEQ ID NOS: 1-5 6 hsa-miR-16-5p (RNA) 7 hsa-miR-15a-5p (RNA) 8hsa-miR-15b-5p (RNA) 9 hsa-miR-195-5p (RNA) 10 hsa-miR-424-5p (RNA) 11hsa-miR-497-5p (RNA) 12 Consensus sequence between SEQ ID NOS: 6-11 13Example of DNA portion of 3′ blocking molecule that targets mir-16.

What is claimed is:
 1. A method of reducing the ability of an unwantedmicro-RNA (miRNA) that is one of a plurality of miRNAs fromparticipating in reverse transcription polymerase chain reactions(RT-PCR), the unwanted miRNA having a 5′ end and a 3′ end, the methodcomprising: (a) annealing a complementary region of a blocking nucleicacid to a binding site at a first end of the unwanted miRNA, wherein thefirst end of the unwanted miRNA is one of the 5′ end or the 3′ end; andwherein the blocking nucleic acid comprises (i) a 5′ end of the blockingnucleic acid and a 3′ end of the blocking nucleic acid; (ii) asingle-stranded complementary region at one of the 5′ end of theblocking nucleic acid or the 3′ end of the blocking nucleic acid, thatanneals with the binding site under stringent conditions; (iii) ahairpin loop forming region adjacent to the complementary region, thehairpin loop forming region having a ligative terminal end; and (iv) afirst blocking moiety linked to the terminal end of the complementaryregion, in which said first blocking moiety cannot serve as a substratefor ligases; (b) ligating an adenylated nucleic acid adapter to a 3′ endof at least one of the plurality of miRNAs, ligating an RNA adapter to a5′ end of at least one of the plurality of miRNAs, or a combinationthereof, wherein step (b) occurs after step (a), and (i) when theblocking nucleic acid is a 3′ blocking nucleic acid, the blockingnucleic acid prevents ligation of the adenylated nucleic acid adapter tothe 3′ end of the unwanted miRNA; and (ii) when the blocking nucleicacid is a 5′ blocking nucleic acid, the blocking nucleic acid preventsligation of the RNA adapter to the 5′ end of the unwanted miRNA; and (c)performing RT-PCR on the plurality of miRNAs, wherein: (i) when theblocking nucleic acid is a 3′ blocking nucleic acid, performing RT-PCRcomprises introducing a reverse transcription primer that hybridizes tothe adenylated nucleic acid adapter; (ii) when the blocking nucleic acidis a 5′ blocking nucleic acid, the RNA adapter provides a primer bindingsite for PCR and performing RT-PCR comprises introducing a primer thattargets the primer binding site; or (iii) a combination thereof.
 2. Themethod of claim 1, wherein annealing is conducted under stringentconditions.
 3. The method of claim 1, wherein annealing is conductedunder maximally stringent conditions.
 4. The method of claim 1,comprising ligating the blocking nucleic acid to the first end of theunwanted miRNA.
 5. The method of claim 1, wherein the first end of theunwanted miRNA is the 5′ end and the complementary region is at the 5′end of the blocking nucleic acid.
 6. The method of claim 1, wherein thefirst end of the unwanted miRNA is the 3′ end and the complementaryregion is at the 3′ end of the blocking nucleic acid.
 7. The method ofclaim 1, comprising ligating the blocking nucleic acid to the first endof the unwanted miRNA, wherein the ligating step is performed using aDNA/RNA ligase.
 8. The method of claim 1, wherein the unwanted miRNA isone or more of: mir-16, mir-15a, mir-15b, mir-195, mir-424, mir-497,mir-486, mir-451, and mir-26.
 9. The method of claim 1, wherein an miRNAlibrary with reduced abundance of unwanted miRNA is produced by step(c).
 10. The method of claim 1, wherein the blocking nucleic acid is a5′ blocking nucleic acid, the method further comprising introducing theRNA adapter and an RNA ligase under conditions to allow the RNA adapterto ligate to a 5′ end of at least one of the plurality of miRNAs priorto step (c).
 11. The method of claim 1, wherein the plurality of miRNAsis isolated from a sample from a subject.
 12. The method of claim 1,wherein the plurality of miRNAs is obtained from an miRNA library. 13.The method of claim 1, wherein the blocking nucleic acid is one of aplurality of blocking nucleic acids present.
 14. The method of claim 1,wherein the blocking nucleic acid comprises one of SEQ ID NOs: 1-4 and13.
 15. The method of claim 1, wherein the first blocking moiety is oneor more of: a nucleotide from which the 3′ hydroxyl group has beenremoved, a nucleotide from which the 5′ phosphate group has beenremoved, phosphoramidite, triethylene glycol, hexa-ethyleneglycol, apropyl group, a propanol group, an organic alcohol, a glycol compound,an inverted deoxynucleotide, a dideoxynucleotide, and an inverteddideoxynucleotide.